I. Introduction Recently experimental and theoretical investigations of the vibrational relaxation dynamics of water and other molecules in condensed phases have attracted a considerable attention. In comparison the vibrational relaxation dynamics of isolated molecules has received much less attention. A theoretical approach based on the adiabatic approximation model of vibrational relaxation has been developed. In this report we shall review the molecular theories and applications of vibrational and electronic SFG. In addition, we shall present recent progress in our theoretical works on both vibrational and electronic SFG spectroscopy. II. Theory 1. Susceptibility Method To treat nonlinear optical processes, the susceptibility method is commonly used. The central problem in the susceptibility method is to calculate the polarization P which is an ensemble average of dipole moment , i.e. P Tr ˆ Tr ˆ (1-1) where ̂ denotes the density matrix of the system. The diagonal and off-diagonal elements of ̂ describe the population and phase (or coherence) of the system, respectively. Notice that the equation of motion for the density matrix ˆ (t ) of the system is given by the stochastic Liouville equation ˆ i i iLˆ 0 ˆ ˆ ˆ iLˆ ' t ˆ Hˆ 0 , ˆ ˆ ˆ Vˆ t , ˆ t i iLˆ'0 ˆ Vˆ , ˆ (1-2) where iLˆ0 iLˆ0 ˆ , and L̂0 and Lˆ (t ) denote the Liouville operators corresponding to the zeroth-order Hamiltonian and Hamiltonian for the interaction Vˆ between the system and radiation field, respectively. ̂ denotes the damping operator describing the relaxation and dephasing of the system. To evaluate P , it is necessary to solve Eq. (1-2). For this purpose, for the weak field case the perturbation method is often used. This can be carried out by regarding L̂ in Eq. (1-2) as a perturbation and solving Eq. (1-2) directly. For SFG, we have to evaluate the second-order polarization P (2) (t ) given by P 2 t Tr ˆ 2 . (1-3) In the dipole approximation, one can write Vˆ (t ) as 2 Vˆ t E ii e it i i li t i (1-4) i and li t e t p t Ti (1-5) where represents the dipole operator, i takes the value 1 or –1 and li (t ) denotes the laser pulse-shape function with pulse duration Ti . The summation over i in Eq. (1-4) describes the situation that in SFG two lasers are involved. Eq. (1-3) is general and can be applied to study difference-frequency and sum-frequency generations. For SFG, for convenience, Eq. (1-3) can be written as P2 t p P2 1 2 exp it p 1 2 P2 1 2 exp it p 1 2 (1-6) where i j k 'l Ei i kk ' E j j lk lk ' i ' 1 T ' k k' l k 'l i i i kl i j 1 T1 1 T2 i j i j k 'k Ei i k 'l E j j lk kk' ' i kl i j 1 T1 1 T2 i j i k' 'k i 1 Ti (1-7) i j k 'l Ei i kk ' E j j lk lk ' ' i k' 'l i 1 Ti i kl i j 1 T1 1 T2 i j * i j k 'k Ei i k 'l E j j lk kk' i kl' i j 1 T1 1 T2 i j i k' 'k i 1 Ti 2 2 i P 1 2 2 where, for example, lk ' ˆ i ll ˆ i k 'k ' representing the initial distributions of the system. In other words, in SFG the coefficient of exp it p 1 2 or exp it p 1 2 should be needed. Here in the double summations over i and j, the i = j terms are to be excluded. One can consider an energy level diagram for SFG as shown in Fig. 1, where g, m and k denote the initial, intermediate and final state manifolds, and 1 and 2 represent the frequencies of the two lasers used in SFG experiments. k 2 1 2 m 1 g Fig. 1. Energy level diagram for SFG. (2) (1 2 ) , According to the definition of the second-order SFG susceptibility 2 1 2 E1 1 E2 2 P2 1 2 (1-8) where , and indicate molecular coordinates x, y and/or z, it is found 2 2 2 2 1 2 1 2 1 1 2 2 1 2 3 where (1-9) 2 1 2 1 i2 2 gm g k m mg km gk ' ' i mg 1 mg i kg 1 2 kg i mg mg km " ikg 1 2 kg' 2 mg mg km kg ' ' i mg 1 mg i kg 1 2 kg i mg mg km " ' 2 mg i kg 1 2 kg * gm kg mk ' ' i gm 1 mg i km 1 2 km gm kg igm 2 mg" ikm 1 2 km' gm kg km ' ' i gm 1 mg i km 1 2 km gm kg igm 2 mg" ikm 1 2 km' * (1-10) (2) (2) (2) (1 2 )2 and (1 2 )3 can be obtained from (1 2 )1 by performing the exchanges (2) (2) (1 2 )2 and (1 2 )3 are negligible and k m and k g , respectively. The expressions for ' mg 1 T1 , will not be produced here. Note that in Eq. (1-10), mg " mg mg 1 T2 , and kg' kg 1 T1 1 T2 . As discussed in previous papers, there are four cases of SFG. They are resonance–off-resonance, off-resonance–resonance, resonance–resonance, and off-resonance–off-resonance cases. 2. Electronic Sum-Frequency Generation A. Resonance–Off-Resonance Case kw 2 1 2 mu 1 gv Fig. 2. Energy level diagram for resonance–off-resonance SFG. In this case, from Eq. (1-10) we obtain 2 1 2 i 2 gm g k m mg ' i mg 1 mg (2-1) gk km km gk kg 1 2 km 1 2 For electronic SFG, we have to make the following changes due to the use of the Born-Oppenheimer approximation, g gv , m mu , k kw where gv, mu and kw denote the vibronic states. It follows that 2 1 2 i 2 gv, mu v kw mu (2-2) gv,kw kw,mu kw,mu gv,kw mu, gv ' i mu , gv 1 mg kw, gv 1 2 kw,mu 1 2 By using the Placzek approximation and the closure relation, kw kw 1 , we obtain, w 2 1 2 i 2 gv, mu v kw mu mu mg gv mu gk km gv mu gk km gv ' i mu , gv 1 mg kw, gv 1 2 kw,mu 1 2 or (2-3) 2 1 2 i 2 gv, mu v kw mu (2-4) mu mg gv gv gm mu ' i mu , gv 1 mg and 2 2 2 1 2 Re 1 2 i Im 1 2 (2-5) where the real part is 1 2 Re 1 2 2 gv, mu v u 1 mu mg gv mu , gv gv gm mu (2-6) 1 '2mu , gv 2 mu , gv and the imaginary part is 1 2 Im 1 2 2 gv, mu v ' mu , gv u mu mg gv gv gm mu (2-7) 1 '2mu , gv 2 mu , gv (2) (1 2 ) is closely related to the electronic absorption Eq. (2-7) indicates that the imaginary part of spectra. In the Condon approximation we obtain, Re 1 2 2 1 2 mg gm gv, mu mu , gv v u 1 gv mu 1 2 mu , gv '2 mu , gv 2 (2-8) and Im 1 2 2 1 2 mg gm gv, mu v u 'mu , gv gv mu 2 '2 mu , gv 1 mu , gv 2 (2-9) Notice that gk km gm k The kg 1 2 gk km 0 gm gm kg gm 1 2 (2-10) g m transition intensity in electronic SFG is determined by the electronic matrix element 2 mg gm rather than mg in the ordinary absorption spectra. B. Off-Resonance–Resonance Case kw 1 2 2 mu 1 gv Fig. 3. Energy level diagram for off-resonance–resonance SFG. We next consider the off-resonance–resonance case. From Eq. (1-10) we find 2 1 2 i 2 gm i g k m kg mg km mg km gk (2-11) ' 1 2 kg mg 1 mg 2 In terms of vibronic states, we find (with the Placzek approximation mu , gv mg ) 2 1 2 i 2 gv, mu i v w m gv gk kw kw , gv ' 1 2 kw , gv kw km mg kw kw km mg gv mg 1 mg 2 (2-12) or 2 1 2 i 2 gv, mu v w gv gk kw kw kg gv ' i kw, gv 1 2 kw , gv (2-13) where km mg kg m mg 1 km mg mg 2 (2-14) In the Condon approximation we find that 2 Re 1 2 and 1 2 gk gv, mu kw, gv kg v w 1 2 kw gv '2 kw, gv 1 2 kw, gv 2 2 (2-15) 2 Im 1 2 1 gk kg gv, mu 2 v w 'kw, gv kw gv 2 kw, gv 1 2 '2kw, gv 2 (2-16) 2 Im 1 2 shows that it is closely related to the absorption spectra for the electronic transition g k with frequency 1 2 . C. Double Resonance Case kw 1 2 2 mu 1 gv Fig. 4. Energy level diagram for double resonance SFG. Next we consider the double resonance case. From Eq. (1-10) we obtain 2 1 2 i2 2 gv, mu v gv,kw mu kw (2-17) mu , gv kw,mu imu,gv 1 mu' ,gv ikw,gv 1 2 kw' ,gv or 2 1 2 i2 gv, mu 2 v mu kw gv gk kw mu mg gv kw km mu i mu , gv ' ' 1 mu , gv i kw , gv 1 2 kw , gv (2-18) In the Condon approximation, 2 1 2 Notice that i2 gk mg km gv, mu 2 v mu kw i gv kw mu gv kw mu mu , gv ' ' 1 mu , gv i kw , gv 1 2 kw , gv (2-19) i2 2 Re 1 2 2 gk mg km gv, mu v u w mu , gv 1 kw, gv 1 2 'mu , gv 'kw, gv (2-20) gv kw kw mu mu gv 2 2 '2 '2 mu , gv 1 mu , gv kw, gv 1 2 kw, gv and i2 2 Im 1 2 2 gk mg km gv, mu v u w 'mu , gv kw, gv 1 2 'kw, gv mu , gv 1 (2-21) gv kw kw mu mu gv 2 2 '2 '2kw, gv mu , gv 1 mu , gv kw , gv 1 2 3. Vibrational Sum-Frequency Generation A. Resonace–Off-Resonance Case ku 2 1 2 gv’ 1 gv Fig. 5. Energy level diagram for resonance–off-resonance SFG. In this case, from Eq. (1-10) we obtain 2 1 2 i 2 gm g k m mg ' i mg 1 mg gk km km gk kg 1 2 km 1 2 (3-1) For vibrational SFG, we have to make the following changes due to the use of the Born-Oppenheimer approximation, g gv, m gv' , k ku It follows that 2 1 2 gv, gv' i 2 v v' ku gv',gv i gv',gv 1 gv' ',gv gv,ku ku , gv' ku , gv' gv,ku ku ,gv' 1 2 ku , gv 1 2 (3-2) Using the Placzek approximation, we obtain 2 1 2 i 2 gv, gv' v v' gv' gg gv i gv', gv 1 gv' ', gv gk kg gk kg gv gv' k kg 1 2 kg 1 2 (3-3) or 2 1 2 i 2 gv' gg gv gv, gv' i v v' gv ', gv 1 gv' ', gv gv gg gv' (3-4) where gg denotes the polarizability of the g electronic state, gk kg gg k kg 1 2 gk kg kg 1 2 (3-5) Both gg and gg should be expanded in terms of normal coordinates Ql, gg Ql Ql 0 0 gg gg l (3-6) and gg Ql Ql 0 0 gg gg l (3-7) Substituting Eqs. (3-6) and (3-7) into Eq. (3-4) yields 2 1 2 i Kl 1 , 2 gv, gv' v v' l gv Ql gv' 2 ' i gv', gv 1 gv ', gv (3-8) where Kl 1 , 2 1 gg gg 2 Ql 0 Ql 0 This indicates that for this vibrational SFG to be non-zero, we should have (3-9) gg 0 Ql 0 (3-10) gg 0 Ql 0 (3-11) and That is, the Ql mode should be both IR and Raman active. 2 The phase of the SFG can now be determined; in this case 1 2 should be written in terms of the real part and the imaginary part. That is, Re 1 2 Kl 1 , 2 gv, gv ' 2 v v' l gv ', gv 1 gv Ql gv ' 2 (3-12) gv ', gv 1 '2gv ', gv 2 and Im 1 2 Kl 1 , 2 gv, gv ' 2 v v' l 'gv ', gv gv Ql gv ' 2 '2 gv ', gv 1 gv ', gv 2 (3-13) 2 2 Notice that Re 1 2 and Im 1 2 satisfy the Kramers-Kroning relation. B. Double-Resonance Case ku 1 2 2 gv’ 1 gv Fig. 6. Energy level diagram for double resonance SFG. In this case, from Eq. (1-10) we find 2 1 2 i2 gm 2 g m k gk mg km img 1 mg' ikg 1 2 kg' In the adiabatic approximation, Eq. (3-15) can be written as (3-14) 2 1 2 i2 2 gv, gv' v v' u gv,ku gv',gv ku , gv' igv',gv 1 gv' ',gv iku ,gv 1 2 ku' ,gv (3-15) or 2 1 2 gv' gg gv i2 gv , gv ' 2 v v' i gv',gv 1 gv' ',gv gv gk ku ku kg gv' i ku , gv 1 2 ku' , gv u (3-16) This implies that this type of SFG consists of the Raman and resonance Raman scattering, gg gv' Ql gv 2 Q i l 0 2 1 2 2 gv, gv' v v' l i gv',gv 1 gv' ',gv gv gk ku ku kg gv' i ku , gv 1 2 ku' , gv u (3-17) or in the Condon approximation, gg gv' Ql gv Ql 0 i 2 0 0 1 2 2 gk kg gv, gv' i gv',gv 1 gv' ',gv v v' l 2 (3-18) gv ku ku gv' ' u i ku , gv 1 2 ku , gv 4. Surface-enhanced Sum-Frequency Generation Similar to SERS (surface-enhanced Raman Scattering), its chemical enhancement can be due to the appearance of charge-transfer bands which originate from the interaction between the adsorbed molecules and metal (like Ag and Au) (See Figs. 4 and 6). 5. Orientation and Euler Angle Transform Till now we focused on the second-order susceptibility, (2), of a single molecule. To correlate these molecular properties to experimental observables, one has to include information of molecular orientation distribution as well as beam angles defined under laboratory frames. Given the molecular coordinates, xyz, laboratory frames, XYZ, and the rotation angles, , in the z-y-z convention (Fig. 7), the Euler rotation matrix between the two coordinate systems is Z z y x Y X Fig. 7. Cartesian coordinates of laboratory frames (XYZ) and molecular axes (xyz) and rotation angles () between the two coordinate systems. RXx R RYx RZx RXz RYy RYz RZy RZz cos cos cos sin sin sin cos cos cos sin sin cos RXy cos cos sin sin cos sin cos sin cos cos sin sin cos sin sin sin cos (5-1) Now we consider the projection of molecular second-order susceptibility onto the laboratory frame. With an orientation average over a group of molecules, the projection is (2) (2) (2) ˆ )(ˆ nˆ ) N lmn N lmn (ˆ lˆ)( ˆ m RIl RJm RKn lmn (5-2) lmn where , and indicate laboratory frames X, Y and/or Z, l, m and n refer to molecular coordinates x, y and/or z, N is the density of molecules at the interface, and is the orientation average. (2) has 27 elements but most of them vanish supposing the molecules have an azimuthally isotropic distribution on the XY plane. For example, on the (0001) surface of ice Ih crystal only seven elements survive and some of them are mutually dependent after the orientation average: (2) ZZZ , (2) (2) XXZ YYZ , (2) ZXX (2) ZXZ (5-3) (2) ZYY (2) YZY . Next we take beam angles into account. Given 1 and 2 the incident beam angles and SFG the output beam angle with respect to the surface/interface normal, the effective susceptibilities at different combination of beam polarizations (s or p) areError! Bookmark not defined. (2) (2) eff ( ssp) sin 1 LYY (SFG ) LYY (2 ) LZZ (1 ) XXZ (5-4) (2) (2) eff ( sps) sin 2 LYY (SFG ) LZZ (2 ) LYY (1 ) XZX (5-5) (2) (2) eff ( ppp) sin SFG sin 2 sin 1 LZZ (SFG ) LZZ (2 ) LZZ (1 ) ZZZ (2) cos SFG cos 2 sin 1 LXX (SFG ) LXX (2 ) LZZ (1 ) XXZ (5-6) (2) sin SFG cos 2 cos 1 LZZ (SFG ) LXX (2 ) LXX (1 ) XZX (2) cos SFG sin 2 cos 1 LXX (SFG ) LZZ (2 ) LXX (1 ) XZ X where the sequence of beam polarization noted here is SFG, 2 and 1. In above equations L’s refer to Fresnel factors which are functions of refractive indices and wavelengths.Error! Bookmark not defined., III. Applications 1. Vibrational SFG Studies A. Early achievements The SFG technique has become available on analyzing surface species since the works conducted by Shen’s group in late 1987. With an optical parametric amplifier tuned around 2800 to 3100 cm -1 with a frequency-doubled Nd:YAG laser, they have successfully recorded IR-Vis SFG spectra of methanol and pentadecanoic acid (PDA) adsorbed on glass and on water surface. By tuning the IR frequency, different vibrational resonance were obtained, and with various combinations of incident beam polarizations (s and/or p), different C-H stretching modes were revealed. They have further analyzed C-H vibration signals of PDA at different adsorption densities, finding the long alkane chains nearly oriented straightly along the normal to the water surface in the liquid-condensed phase at high density.Error! Bookmark not defined. Following these pioneering works, numerous applications of vibrational SFG spectroscopy emerged. One major category of experiments focused on the orientations of functional groups of organic polymers adsorbed on solid surfaces. Aliphatic and aromatic C-H stretching modes from different chemical species could be distinguished by resonance frequencies, and their orientations could be deduced from signal intensities recorded under different combinations of incident/output beam polarizations. The results showed some substrate-dependence of the adsorbate orientation. For example, the phenyl groups in polystyrene have an average tilt angle of ~20° at the polystyrene-air interface when the polymer is adsorbed on sapphire, and ~57° on oxidized silicon substrate.Error! Bookmark not defined.,Error! Bookmark not defined. The SFG technique could identify both chemical species and orientations at the same time, which combines the advantages of other surface-detecting techniques such as Raman scattering, atomic force microscopy, and X-ray spectroscopy. B. Water surfaces (water/vapor interfaces) The configuration and related properties of water surface has long been a crucial topic but it lacks even a generally acceptable conclusion until the infrared-visible SFG experiments directed by Shen’s group in 1990s. In a simple model that described / Q along the OH bond and / Q with a cylindrical symmetry, only two components of hyperpolarizabilities were non-vanishing. The tilt angle of dangling OH on the pure water (2) (2) surface could then be deduced from the ratio of XZX and XXZ which were measured with ssp and sps polarization combinations, respectively. It led to a maximal tilt angle of 38°, and furthermore, a minimal coverage of 20% of dangling OH over the water monolayer.Error! Bookmark not defined. They have further recorded ssp, sps, and ppp signals for dangling OHs and given a more comprehensive analysis how the maximal tilt angle would affect the signal intensity. Richmond’s group, on the other hand, investigated SFG spectra of not only pure H2O but also pure D2O and their mixture. In addition to liquid water, Shen’s group paid attention to the surface of ice which, in principle, has a more regular structure and could yield more solid intepretation.Error! Bookmark not defined., The surface structure and the melting phenomenon of the (0001) face of ice Ih were studied. It was supposed each layer of water molecules in the ice crystal were bonded by the tetrahedral hydrogen bonding, and the topmost layer had free (dangling) OH bonds pointing outward with a tilt angle of . For this ice crystal structure, the non-vanishing elements of the second-order susceptibility reduced to three [Eq. (5-3)], and all three elements could be determined from SFG signals with ssp, sps, and ppp polarization combinations [Eqs. (5-4) to (5-6)]. In the temperature-dependence measurements from 173K up to the bulk melting point 273K, the free OH peak at 3695 cm–1 gradually diminished while the broad bonded OH band red-shifted from ~3150 cm–1 to ~3250 cm–1. The decrease of the free OH peak intensity represented a larger tilt angle and a more disordered structure as the temperature increased. Assuming a simple flat orientation distribution at each , it was found the maximum tilt angle to be 0° at low temperature while ~60° near the melting point. A more surprising founding was that around 273K, the supercooled liquid water surface was even more ordered than ice (cf. Fig. 9 in Ref. Error! Bookmark not defined.). This gave a remark on the semi-ordered, ice-like structure on the water surface, which has raised debate over decades and has not come to consensus yet. C. Phase-sensitive detections In early IR-Vis/UV SFG experiments only (2) 2 could be obtained from signal intensities. Neither Re[ (2) ] nor Im[ (2) ] could be measured directly; instead, they were fitted from (2) 2 profiles and might yield significantly different interpretation (Fig. 8). As early as in 1990 Superfine et al. tried to determine the “absolute phase” of second-order susceptibility by investigating the relative sign of dipole moment derivative and polarizability derivative with respect to the vibrational normal mode. This technique became mature in the 2000s and was applied on the water interface geometry.Error! Bookmark not defined., Fig. 8. A demonstration of quite different fittings to the same (2) 2 spectrum. Adopted from Ref. Error! Bookmark not defined.. In vibrational SFG spectroscopy, the sign of Im[ (2) ] is determined by [cf. Eq. (3-10)], and Q Q hence in the case of water surface, this sign can reflects directions of OH bonds supposing the OH stretching modes are well decoupled from each other, i.e. performed as local modes. This decoupling is practical since each OH bond faces somewhat different environment constructed by surrounding water molecules. As a result, OH bonds pointing outward from the surface are positive, and those pointing into the bulk are negative, in the Im[ (2) ] spectra. Tian and Shen have carried out phase-sensitive vibrational SFG spectroscopy experiments for pure H2O, and later on pure D2O as well as H2O/D2O mixture at different ratios. On the surface of pure H2O, the dangling OH bonds, as expected, had a sharp positive peak at 3690 cm1. The hydrogen-bonded OH bonds, however, present one negative band centered at 3450 cm1 and one positive band centered at 3100 cm1 (Fig. 9). The same features were also observed by Tahara and coworkers. It should be noticed that the positive band could not be correctly resolved previously by just fitting real and imaginary parts to the (2) 2 spectrum.Error! Bookmark not defined. Comparing these band frequencies to IR absorption bands of bulk ice and liquid samples, it was concluded that OHs in the topmost layer DAA, DDA, and DDAA (Fig. 10, where D and A represent hydrogen donor and hydrogen acceptor, respectively, in the hydrogen-bonding network) molecules loosely donor-bonded to molecules below contributed to the “liquid-like” negative band, while OHs of tetrahedrally bonded DDAA molecules in lower layers contributed to the “ice-like” positive band through their donor-bonding to upper DAA and DDA molecules. This supported the idea that the surface layer of water is relatively disordered, while the lower layers have some ordered (to a certain extent) ice-like structure.Error! Bookmark not defined.,Error! Bookmark not defined. Fig. 9. Phase-sensitive vibrational SFG spectra of water surface in the OH stretching region. Adopted from Ref. Error! Bookmark not defined.. Fig. 10. Configurations of water molecules categorized by hydrogen donating/accepting in the hydrogen-bonding network. D. Theoretical and computational approaches Case 1: Organic molecules adsorbed on graphene surface Graphene is a novel material due to its unique geometric structure and hence related properties such as electric potential, conductivity, etc. It raises interest as well in the adsorption behavior of this substrate, e.g. the orientations of organic polymers adsorbed on the graphene surface, and people have initiated SFG experiments to study this topic. From the theoretical point of view, we have carried out a preliminary SFG simulation of styrene–graphene adsorption system. In our approach, the first step was to construct molecular models of ethylbenzene, styrene monomer, and polymerized styrene oligomers (up to 4 units) adsorbed on a (finite-sized) graphene sheet. It followed geometric optimization of styrene molecules by DFT computations and it showed the phenyl groups tended to “stand” rather than “lay down” on the graphene surface. The adsorption energies were weak but not negligible. With calculated dipole derivatives and polarizability derivatives the vibrational SFG spectra of phenyl and alkyl CH stretching modes were simulated with different polarization combinations. These results, some of which are demonstrated in Figs. 11 and 12, have provided the first theoretical prediction to the graphene-based adsorption system and is waiting for experimental investigation. Fig. 11. Optimized structures and stabilization energies (in kcal/mol) of ethylbenzene–graphene and styrene oligomer–graphene systems calculated by B3LYP/6-31G(d). Adopted from Ref. Error! Bookmark not defined.. 8 ssp sps (5x) ppp etb(a) 6 10 sty2(a) ssp sps (5x) ppp 6 4 SFG Intensity (arb. unit) 4 2 0 0 etb(b) 3 ssp sps (10x) ppp 10 20 8 sty2(b) ssp sps (5x) ppp 6 4 1 0 0 etb(c) 3 ssp sps (10x) ppp ssp sps (5x) ppp 0 8 sty2(c) ssp sps (5x) ppp sty3(c) ssp sps (5x) ppp 6 2 4 1 0 2800 sty3(b) 2 1 2 0 2900 3000 3100 3200 2800 -1 Wavenumber (cm ) Fig. 12. ssp sps (5x) ppp 0 2 5 2 sty3(a) 10 2 15 30 8 0 2900 3000 3100 -1 Wavenumber (cm ) 3200 2800 2900 3000 3100 3200 -1 Wavenumber (cm ) Simulated SFG spectra of ethylbenzene– and styrene-oligomer–graphene systems. corresponding conformers are shown in Fig. 11. The Adopted from Ref. Error! Bookmark not defined.. Case 2: Molecular orientation on the water surface (liquid water/vapor interface) Since the first report of the water surface SFG experiment, theoreticians started to make efforts on interpreting and simulating the spectrum. A main approach is to calculate the second-order susceptibility by a time correlation function of polarizability tensors and dipole vectors obtained from molecular dynamics (MD) simulation. The major problem in this approach is that the accuracy of the MD potential field definitely affects the accuracy of simulated spectra, in which the positive band centered at ~3100 cm1 (Refs. Error! Bookmark not defined.Error! Bookmark not defined., cf. Fig. 9) in Im[ (2) ] has never been reproduced until very recently. Morita and Tahara considered the dipoleinduced-dipole interaction in a pair of water molecules and attributed the low-frequency positive band to the strong hydrogen bonding between the two; however, the direction of induced dipole is questionable. Skinner and coworkers have constructed a potential which explicitly describes three-body interactions and finally obtained the positive band, although the peak positions still required modification. We would like to treat this problem with a more quantum chemical approach based on DAA, DDA, and DDAA configurations. By randomizing orientations and distances of the surrounding three or four molecules, the explicit, averaged dipole and polarizability derivatives of the center water molecule at each configuration could be calculated. The overall simulated SFG spectrum could reproduce detailed features quite well, and the arrangement of report is in the process. 2. Electronic SFG studies A. Recent experiments In addition to VSFG, Tahara and co-workers have recently made heterodyne-detected electronic sum-frequency generation (ESFG) applicable. In their experiments, two visible/near-infrared beams, one was 795 nm and the other was tunable between 540 nm and 1.2 m, were guided to the sample where the sum frequency was tuned resonant with an electronic state. By this means they have resolved the orientation of surfactant-like molecules floating on water surface. Following that they have studied the pH spectrometry of an air/cationic surfactant/water interface by the same technique, obtaining an insight to the acid-base equilibrium, and furthermore, to the pH value of water surface, where the value is lower than the bulk by 1.7. It turns out that electronic SFG is helpful to solve this long-debated issue; however, it should be noticed that a solid theoretical framework to interpret these excellent experimental results is still in demand. B. Recent theoretical approaches We have chosen 4-methyl-7-hydroxycoumarin (MHC), a pH-indicator surfactant molecule analogous to which Tahara’s group used in their experiments,Error! Bookmark not defined. to provide a theoretical support to ESFG spectroscopy. A loose optimization showed that the MHC molecule tends to float on the water surface (simulated by a cluster of ~50 water molecules) with its oxygen atoms immersed into water while methyl group pointing outward (Fig. 13). Under the off-resonance–resonance scheme, the dominant transition occurred from the ground state (1 1A') to the first excited state (2 1A'). The corresponding transition dipole moments, two-photon matrix elements, and Franck-Condon integrals were calculated according to Eqs. (2-13) to (2-16) and the SFG spectrum was then simulated (Fig. 14). This preliminary result is the first theoretical ESFG spectrum to our knowledge, and it may be further improved by including more detailed factors like non-Condon contributions, more definite orientation distributions, and explicit solve effects. Fig. 13. MHC anion floating on the surface of a small water cluster, where red and dark gray spheres represent oxygen and carbon atoms, respectively. 0.005 eff,sps (2) (arb. unit) 0.004 0.003 0.002 0.001 0.000 Im[ Re[ -0.001 (2) ] ] (2) -0.002 -0.003 260 280 300 320 Wavelength (nm) 340 360 Fig. 14. Simulated electronic SFG spectrum of MHC with the sps polarization. References 1. M. Hayashi and S. H. Lin, “Molecular theory of sum-frequency generations and its applications to study molecular chirality”, in Advances in Multi-photon Processes and Spectroscopy, S. H. Lin, A. A. Villaeys, and Y. Fujimura, Eds; World Scientific: Singapore, 2004; Vol. 16, pp 307-422. 2. A. J. Moad and G. J. Simpson, “A Unified Treatment of Selection Rules and Symmetry Relations for Sum-Frequency and Second Harmonic Spectroscopies”, J. Phys. Chem. B 108, 3548 (2004). 3. X. Wei, P. B. Miranda, C. Zhang, and Y. R. Shen, “Sum-frequency spectroscopic studies of ice interfaces”, Phys. Rev. B 66, 085401 (2002). 4. X. Zhuang, P. B. Miranda, D. Kim, and Y. R. Shen, “Mapping molecular orientation and conformation at interfaces by surface nonlinear optics”, Phys. Rev. B 59, 12632 (1999). 5. J. H. Hunt, P. Guyot-Sionnest, and Y. R. Shen, “Observation of C-H Stretch vibrations of Monolayers of Molecules Optical Sum-Frequency Generation”, Chem. Phys. Lett. 133, 189 (1987). 6. P. Guyot-Sionnest, J. H. Hunt, and Y. R. 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PUBLICATION LIST (JOURNALS, CONFERENCES, BOOKS, BOOK CHAPTERS, etc.) Journal paper summary 1. “Absorption and fluorescence spectra of the neutral and anionic green fluorescent protein chromophore: Franck-Condon simulation”, Tsung-wei Huang, Ling Yang, Chaoyuan Zhu, Sheng Hsien Lin, Chemical Physics Letters (ISSN: 0009-2614), 541(10), 110-116 (2012) 2. “Analysis of Initial Reactions of MALDI based on Chemical Properties of Matrices and Excitation Condition”, YH Lai, CC Wang, CW Chen, BH Liu, SH Lin, YT Lee, YS Wang, J PHYS CHEM B (ISSN: 1520-6106), 116(32), 9635-9643 (2012) 3. “Ionization/Dissociation Processes of Methyl-Substituted Derivates of Cyalopentanone in Intense Femtosecond Laser Field”, Q Wang, D Wu, D Zhang, M Jin, F Liu, H Liu, Z Hu, D Ding, YA Dyakov, H Mineo, Y Teranishi, SD Chao, AM Mebel and SH Lin, Chemical Physics (0301-0104), (2012/12) 4. “Quantums Switching of pi-Electron Rotations in a Nonplanar Chiral Molecule by Linearly Polarized UV Laser Pulses”, H Mineo, M Yamaki, Y Teranishi, M Hayashi, SH Lin and Y Fujimura, Journal of the American Chemical Society (ISSN: 0002-7863), 134(35), 14279-14282 (2012) 5. “Localized Emitting State and Energy Transfer Properties of Quadrupolar Chromophores and (Multi) Branched Derivatives”, LY Yan, XD Chen, QG He, YY Wang, XF Wang, QJ Guo, FL Bai, AD Xia, D Aumiler, V Silviie, SH Lin, Journal of Physical Chemistry A (ISSN: 1089-5639), 116(34), 8693-8705 (2012) 6. “Semiclassical Instanton Theory of Nonadiabatic Reaction Rate Constant. I. Formulation”, Y Teranishi, H Nakamura, SH Lin, AIP Conference Proceedings (ISSN: 0094-243X), 1456, 119-130 (2012) 7. “Thermodynamics of protein folding using a modified Wako-Saitô-Muñoz-Eaton model ”, Min-Yeh Tsai, Jian-Min Yuan, Yoshiaki Teranishi, Sheng Hsien Lin, Journal of Biological Physics (ISSN: 0092-0606), 38(4), 543-571 (2012) 8. “Quantum Chemical Calculation of Intramolecular Vibrational Relaxation and Vibrational Energy Transfer of Water Clusters Corresponding”, Sheng-Hsien Lin, Ying-Li Niu, Ran Pang, Chao-Yuan Zhu, Michitoshi Hayashi, Yuichi Fujimura, Yuen-Ron Shen, Chemical Physics Letters, 9. “Visible-light Photocatalytic Activity of Ni-doped TiO2 from ab initio Calculations”, Y.-M. Lin, Z.-Y. Jiang, C. Zhu, X.-Y Hu, X.-D. Zhang, J. Fan, Mater. Chem. Phys., 133, 746 (2012) 10. “Highly hydrated Nafion/activated carbon hybrids”, H.-C. Chien, L.-D. Tsai, A. Kelarakis, C.-M. Lai, J.-N. Lin, J. Fang, C. Zhu, F.-C. Chang, Polymer, 53, 4927-4930 (2012) 11. “Enhanced optical absorption and photocatalytic activity of anatase TiO2 through (Si, Ni) codoping”, Y.-M. Lin, Z.-Y. Jiang, C. Zhu, X.-Y. Hu, X.-D. Zhang, H.-Y. Zhu, and J. Fan, Appl. Phys. Lett. (ISSN: 0003-6951), 101, 062106 (2012) 12. “Recent developments in radiationless transitions”, Y. Niu, C.-K. Lin, L. Yang, J. Yu, R. He, R. Pang, C. Zhu, M. Hayashi, and S. H. Lin, Prog. Chem., 26, 87 (2012) 13. “Recent Developments in Theoretical Chemistry”, Y. L. Niu, M. Yamaki, C. Zhu, M. Hayashi, Y. Fujimura, and S. H. Lin, Bulletin of Association of Asia Pacific Physical Societies (ISSN: 0218-2203), 22, 12-16 (2012) (IF: 1.3) 14. “Coherent ring currents in chiral aromatic molecules induced by linearly polarized UV laser pulses”, Manabu Kanno, Hirohiko Kono, Hirobumi Mineo, Sheng Hsien Lin, and Yuichi Fujimura, Key Engineering Materials(Materials and Applications for Sensors and Transducers II-Proceedings of the 2nd International Conference), 543, 381-384 (2013) (ISSN: 1662-9795) 15. “Coherent π-electron dynamics of (P)-2,2_-biphenol induced by ultrashort linearly polarized UV pulses: Angular momentum and ring current”, H. Mineo, S. H. Lin and Y. Fujimura, J. Chem. Phys., 138(7), 074304(16) (2013)(ISSN: 0021-9606) 16. “Neutral Fragmentation Paths of Methane Induced by Intense Ultrashort IR Laser Pulses: Ab Initio Molecular Orbital Approach”, Shiro Koseki, Noriyuki Shimakura, Yoshiaki Teranishi, Sheng Hsien Lin, and Yuichi Fujimura, J. Phys. Chem. A (ISSN: 1089-5639), 117(2), 333-341, (2013) 17. “Anharmonic Rice-Ramsperger-Kassel-Marcus (RRKM) and product branching ratio calculations for the partially deuterated protonated water dimmers: Dissociation and isomerization”, Di Song, Hongmei Su, Fan-ao Kong, and Sheng-Hsien Lin, J. Chem. Phys. 138(10), 104301(7), 2013 (ISSN: 0021-9606) 18. “C/B codoping effect on band gap narrowing and optical performance of TiO2 photocatalyst: a spin-polarized DFT study”, Y. Lin, Z. Jiang, C. Zhu, X. Hu, X. Zhang, H. Zhu, J. Fan, and SH Lin, J. Mater. Chem. A (ISSN: 0959-9428), 1, 4516-4524 (2013) 19. "Solvent Effects on Vibrational Normal Modes of Solute Molecule: Franck-Condon Simulation of the X <sup>1</sup>A<sub>g</sub>↔ A<sup>1</sup>B<sub>1u</sub> Absorption and Fluorescence Spectra of Perylene in Benzene Solution", Yang, Ling; Zhu, Chaoyuan; Yu, Jian-Guo; Lin, Sheng Hsien, Journal of Physical Chemistry Letters, Manuscript ID: jz-2013-00024y (2013) 20. “Noncovalent interaction and its influence on excited-state behavior: A theoretical study on the mixed coaggregates of dicyanonaphthalene and pyrazoline”, Y. Dai, M. Guo, J. Peng, W. Shen, M. Li, R. He, C. Zhu, and SH Lin, Chem. Phys. Lett. (ISSN: 0009-2614), 556, 29, 230-236 (2013) 21. “Theoretical Study on Structure and Sum- Frequency Generation (SFG) Spectroscopy of Styrene-Graphene Adsorption System”, C-K. Lin, C-C. Shih, Y. Niu, M-Y. Tsai, Y-J. Shiu, C. Zhu, M. Hayashi, and SH Lin, J. Phys. Chem. C (ISSN: 1932-7447), 117, 1754-1760 (2013) 22. "Solvent Effects on Vibrational Normal Modes of Solute Molecule: Franck-Condon Simulation of the X <sup>1</sup>Ag↔ A<sup>1</sup>B<sub>1u</sub> Absorption and Fluorescence Spectra of Perylene in Benzene Solution", Yang, Ling; Zhu, Chaoyuan; Yu, Jian-Guo; Lin, Sheng Hsien, Journal of Physical Chemistry A, Manuscript ID: jp-2013-016382 (2013) 23. "Explore the Role of Varied-Length Spacers in the Charge Transfer: A Theoretical Investigation on the Pyrimidine-Bridged Porphyrin Dyes", Guo, Meiyuan; Li, Ming; Dai, Yulan; Shen, Wei; Peng, Jing-dong; Zhu, Chaoyuan; Lin, Sheng Hsien; He, Rongxing, Journal of Physical Chemistry C, Manuscript ID: jp-2013-01293h (2013) 24. “The Foldon behavior of Cytochrome c as revealed by Molecular Dynamics Simulations and Some Experimental Evidences”, M. Y. Tsai and S. H. Lin, in preparation 25. “Molecular Dynamics Insight into the Thermodynamics of a Beta-Hairpin Peptide”, M. Y. Tsai, J. M. Yuan, M. Yamaki and S. H. Lin, in preparation 26. "Theoretical Formulation and Simulation of Electronic Sum-Frequency Generation (SFG) Spectroscopy", Lin, Chih-Kai; Hayashi, Michitoshi; Lin, Sheng Hsien, Journal of Physical Chemistry C (Submission, Manuscript ID: jp-2013-07881a) (2013) Workshop summary 1. “非平面キラル芳香分子(P)-2,2’BIPHENOL における光誘起Π電子運動の 量子スイッチ ”, HIROBUMI MINEO, MASAHIRO YAMAKI, YOSHIAKI TERANISHI, MICHITOSHI HAYASHI, SHENG-HSIEN LIN, AND YUICHI FUJIMURA, 分子科学討論会, 東京大學 (2012/09/18-21) 2. 14. “線形紫外レーザーパルス 誘起非平面キラル分子におけるΠ電子回転 ダイナミクスの 理論的研究: 角運動量 環電流”, HIROBUMI MINEO, SHENG-HSIEN LIN AND YUICHI FUJIMURA, 分子科学討論会, 東京大學 (2012/09/18-21) “RECENT PROGRESS IN MOLECULAR PHOTOCHEMISTRY”, Y. L. NIU, R. PAUG, L YANG, C. K. LIN, H. MINEO, C. Y. ZHU, S. D. CHUO, Y. FUJIMURA, H. HAYCSHI AND S. H. LIN*, THE 7TH TAIWAN-JAPAN BILATERAL SYMPOSIUM ON ARCHITECTURE OF FUNCTIONAL ORGANIC MOLECULES, HSINCHU, NCTU (2012/10/21-24) 4. “DISCUSSION AND INVESTIGATIONS OF MOLECULAR VIBRATIONAL DYNAMICS”, SHENG-HSIEN LIN, 德國慕尼黑工業大學化學院物理與理論化學研究所(FAKULTÄT FÜR CHEMIE/INSTITUT FÜR PHYSIKALISCHE UND THEORETISCHE CHEMIE/TECHNISCHE UNIVERSITÄT MÜNCHEN/GERMANY), (2012/11/18-27) 5. “MOLECULAR THEORY OF SUM-FREQUENCY GENERATION”, SHENG-HSIEN LIN, 理論計算化學研討 會, 台科大化工系, TAIPEI (2013/01/18-19) 6. “THEORY AND APPLICATION OF INTRAMOLECULAR VIBRATIONAL RELAXATION(共同討論及研究分 子振動動態機制)”, SHENG-HSIEN LIN, ARIZONA STATE UNIVERSITY/ PHOENIX AND UNIVERSITY OF OKLAHOMA/ NORMAN, USA (2013/02/09-22) 7. “”, SHENG-HSIEN LIN, 中國/重慶/西南大學化學化工學院, 廈門/廈門大學固體表面物理化學國 家重點實驗室及北京/中國科學院化學所 (2013/04/25-05/04) 8. “APPLICATION OF INTENSE LASERS TO BIO-ORGANIC MASS SPECTROMETRY”, YUICHI FUJIMURA AND SHENG-HSIEN LIN, INVITATION TO ATTEND THE SHORT PULSE STRONG FIELD LASER PHYSICS INTERNATIONAL SYMPOSIUM HONOURING SEE LEANG CHIN, CENTER D’OPTIQUE, PHOTONIQUE ET LASER, UNIVERSIÉ LAVAL, QUÉBEC, CANADA (2013/05/20-26) 9. “THEORY AND APPLICATIONS OF SUM-FREQUENCY GENERATIONS AND SURFACE-ENHANCED SFG”, C. K. LIN, M. HAYASHI, L. YANG, C. Y. ZHU, Y. FUJIMURA AND S. H. LIN, 6TH WCTCC, 第六屆 世界 華人理論及計算化學研討會(THE 6TH WORLDWIDE CHINESE THEORETICAL AND COMPUTATIONAL CHEMISTRY CONFERENCE (WCTCC)), 24-28 JUNE, 2013, TAMKANG UNIVERSITY, TAIWAN (2013/06/24-28) 10. “NEW DEVELOPMENTS IN BIOLOGICAL PHYSICAL CHEMISTRY AT MOLECULAR LEVEL”, SHENG-HSIEN LIN, CONFERENCE IN HONOUR OF THE 90TH BIRTHDAY OF RUDOLPH MARCUS FUNDAMENTALS IN CHEMISTRY & APPLICATIONS (紀念魯道夫•馬庫斯教授在基礎化學及應用的第 90 個生日研討會), 22-24 JULY, 2013, NANYANG TECHNOLOGICAL UNIVERSITY, SINGAPORE (2013/07/21-25) Books 1. Wolf Prize in Chemistry: An Epitome of Chemistry History in 21th Century and Beyond, Lou-Sing Kan and Sheng-Hsien Lin (World Scientific Publishing, 2012) 2. Advances in Multiphoton Processes and Spectroscopy, Vol. 21, S.H. Lin, A.A. Villaeys and Y. Fujimura (World Scientific Publishing, 2012) 3. “Application of Density Matrix Method to Ultrafast Processes”, YL Niu, CK Lin, CY Zhu, H Mineo, SD Chao, Y Fujimura, M Hayashi and SH Lin, Progress in Theoretical Chemistry and Physics, SPi Technologies India Private Ltd., Manatec Towers, (2013) 4. Theory and Applications of Sum-Frequency Generations”, C. K. Lin, L. Yang, M. Hayashi, C.Y. Zhu, Y. Fujimura, Y. R. Shen, and S.H. Lin∗, JCCS (Journal of the Chinese Chemical Society), No.:jccs.201300416 (Accepted, in the Book of Advances in Multi-Photon Processes and Spectroscopy, Vol. 16) (2013/08/15) 3.