RAPID COMMUNICATIONS IN MASS SPECTROMETRY
Rapid Commun. Mass Spectrom. 13, 1515–1521 (1999)
Internal Energy Content of n-Butylbenzene,
Bromobenzene, Iodobenzene and Aniline
Molecular Ions Generated by Two-photon
Ionization at 266 nm. A Photodissociation Study
Oh Kyu Yoon1, Wan Goo Hwang1, Joong Chul Choe2 and Myung Soo Kim1*
1
National Creative Research Initiative Center for Control of Reaction Dynamics and Department of Chemistry, Seoul National
University, Seoul 151–742, Korea
2
Department of Chemistry, University of Suwon, Suwon 440–600, Korea
A technique to investigate photodissociation kinetics on a nanosecond time scale has been devised for
molecular ions generated by multiphoton ionization (MPI) using mass-analyzed ion kinetic energy
spectrometry. The branching ratio or rate constant has been determined for the photodissociation of the nbutylbenzene, bromobenzene, iodobenzene, and aniline molecular ions generated by MPI at 266 nm. The
ion internal energies have been estimated by comparing the measured kinetic data with the previous energy
dependence data. The analysis has shown that only those molecular ions generated by two-photon ionization
contribute to the photodissociation signals. Around half of the available energy has been found to remain as
molecular ion internal energy in the two-photon ionization process. Copyright # 1999 John Wiley & Sons,
Ltd.
Received 26 April 1999; Revised 14 May 1999; Accepted 18 May 1999
A variety of techniques have been used to investigate
unimolecular dissociation of ions. Direct measurements of
rate constant and kinetic energy release provide invaluable
information for understanding ionic dissociation dynamics.
State-selective experimental results would be the most
useful for the theoretical analysis of the dissociation
process.1 However, such data are difficult to obtain,
especially for ionic cases. Obtaining energy-selective data
is not as difficult. These data, which are the averages over
many quantum states, are usually compared with statistical
theories such as the Rice Ramsperger Kassel Marcus
(RRKM) theory2 to gain dynamical insight into the process.
One of the most powerful techniques to measure dissociation rates of ions with well-defined internal energy is
photoelectron-photoion coincidence spectrometry (PEPICO).3–9 Rates of dissociation occurring on the microsecond
time scale can be determined reliably using this technique.
Dissociations occurring on a time scale longer than a
microsecond have been investigated by photodissociation
ion cyclotron resonance spectrometry.10 In this laboratory, a
technique has been devised to measure photodissociation
rate constants and kinetic energy release distributions on the
nanosecond time scale using mass-analyzed ion kinetic
energy spectrometry (MIKES).11–15
Multiphoton ionization (MPI) has also been used to
measure the dissociation rates on the microsecond time
scale.16–19 Reports of rate constants measured by MPI have
been limited to a few ionic systems such as the molecular
*Correspondence to: M. S. Kim, National Creative Research Initiative
Center for Control of Reaction Dynamics and Department of
Chemistry, Seoul National University, Seoul 151–742, Korea.
Contract/grant sponsor: CRI, the Ministry of Science and Technology,
Republic of Korea.
CCC 0951–4198/99/141515–07 $17.50
ions of benzene,16 chlorobenzene17,18 and aniline.19 In a
standard MPI experiment the mechanism for the production
of fragment ions is often complicated.20 Molecular ions
generated by MPI may absorb further photon(s) and
dissociate to fragment ions (MPID process). Or, neutral
fragments may be formed by multiphoton absorption, which
are then ionized by further absorption of photon(s) (MPDI
process). Additional complications of this technique arise
from the fact that it is difficult to control the number of
photons absorbed. An extremely low level of laser intensity
can be used to avoid the absorption of more photons than
intended. Then, the nonlinear intensity dependence of the
technique often results in extremely poor signal levels,
which renders the technique useless. Theoretically, the main
drawback of this technique in the study of ion dissociation
dynamics is the uncertainty in the internal energy content of
the molecular ions generated by MPI. This occurs because
the photoelectrons ejected carry a range of kinetic energy.
In this paper, we report a new technique to study the
photodissociation of molecular ions generated by MPI on
the nanosecond time scale. The two technical difficulties
mentioned above are more or less avoided by separating the
ionization and dissociation steps, spatially and temporally.
To discover the influence of the third factor, namely that
molecular ions with a range of internal energy is generated
by MPI, the present technique has been applied to ionic
systems with well-established kinetic information. These
include the m/z 91/92 branching ratio in the dissociation of
the n-butylbenzene molecular ion5,12,21 and the rate-energy
relations for the losses of Br, I, and HNC from molecular
ions of bromobenzene,6,7,13,22,23 iodobenzene,8,14,24 and
aniline,9 respectively. Based on the experimental findings,
the overall utility of the present technique for the ion
dissociation study will be discussed.
Copyright # 1999 John Wiley & Sons, Ltd.
1516
INTERNAL ENERGY CONTENT IN TWO-PHOTON IONIZATION
Figure 1. Schematic diagram of the VG ZAB-E mass spectrometer
modified for MPI-PD study.
EXPERIMENTAL
A double-focusing mass spectrometer with reversed geometry (VG ZAB-E; Micromass Plc, Manchester, UK) was
used in this work with some modification (Fig. 1). The
fourth harmonic (266 nm) from an Nd:YAG laser (QuantaRay GCR-150, MPI laser supplied by Spectra Physics
Lasers Inc, Mountain View, USA) was irradiated into a
homemade MPI source along the z-axis (corresponding to
the direction of the magnetic field in the mass spectrometer). The spot size of the laser beam was 10 mm at the
focus. The laser pulse energy was 10–15 mJ, pulse duration
was 4–5 ns, and repetition rate was 50 Hz. In a typical
experiment, the ion source was maintained at room
temperature. The source temperature was raised to 200 °C
for the temperature dependence study. Sample was
introduced into the source via a septum inlet. Sample
pressure in the source was 10ÿ4 Torr. Variation in the
sample pressure by an order of magnitude did not affect the
experimental results indicating that bimolecular processes
were not important.
The ions generated by MPI were accelerated to 8 keV.
MPI mass spectra were obtained under the normal doublefocusing conditions. To observe photodissociation, a molecular ion beam was selected by the magnetic sector and
then crossed perpendicularly with another Nd:YAG laser
(Continuum Powerlite 6050, PD laser Continuum, Sauta
Clara, CA, USA) inside an electrode assembly located near
the intermediate focal point of the instrument.13 The PD
laser beam was irradiated along the z-axis. A Pellin-Broca
prism (CVI Inc, Albuquerque, NM, USA) was employed to
separate the third (355 nm) or fourth (266 nm) harmonics
from the unconverted visible beam. Laser pulse duration
was 6–8 ns and repetition rate was 50 Hz. Laser pulse
energy was reduced to 0.1 mJ using a half-wave plate and
a Rochon (CVI Inc.) prism. Then, the beam was focused by
a cylindrical lens with a focal length of 30 cm.
The width of the laser beam along the ion-optical axis is
estimated to be smaller than 60 mm in the laser beam - ion
beam crossing region. Photodissociation (PD) yield was
measured as a function of the laser beam intensity. The PD
laser intensity was set such that the multiphoton process was
not important.
The translational kinetic energy of fragment ions
produced by photoexcitation was analyzed by the electric
sector. Recording the kinetic energy of fragment ions
generated by the dissociation of mass-selected molecular
Rapid Commun. Mass Spectrom. 13, 1515–1521 (1999)
ions is called mass-analyzed ion kinetic energy spectrometry (MIKES). A schematic diagram for data acquisition is
shown in Fig. 1. It takes a few tenths of a microsecond for
the molecular ions generated in the MPI source to arrive at
the electrode assembly where photodissociation occurs.
Hence, for example, a time delay of 28 ms between the
photoionization (PI) and PD laser pulses was used in the
case of the n-butylbenzene molecular ion accelerated to
8 keV. The delay generator (SRS DG535 Standford
Research Systems, Sunnyvale, CA, USA) also triggers the
boxcar integrator (SRS SR250). The boxcar analog output
was sent to a personal computer, which serves dual purposes
of acquiring a MIKE spectrum and triggering the electronics
for scanning the electric sector voltages. To improve the
quality of a MIKE or mass spectrum, signal averaging was
carried out for repetitive scans.
The principle used to investigate photodissociation
kinetics on the nanosecond time scale is similar to the
previous technique using a continuous-wave Ar ion
laser.11–15 To determine the rate constant, high voltages
are applied to some electrodes such that an electric field is
present in the dissociation region. With the electrode
assembly employed previously,13 two different modes of
operation are possible. The length of the field region is 1cm
in the short-field mode, which was used in the study of
bromobenzene and iodobenzene ions. In the case of the
aniline molecular ion which dissociates more slowly, the
long-field mode with a 4cm field length was used. The
translational kinetic energy of a fragment ion after exiting
the field region depends on the position of its formation.
Hence, the time dependence of dissociation is reflected in
the MIKE spectrum. The method to analyze the MIKE
spectrum to obtain the PD rate constant or its distribution is
well established.11,13 A rate constant in the range of
1 107–8 108 sÿ1 can be determined reliably with the
present method.
In the study of the aniline molecular ion, the unimolecular
dissociation or metastable ion decomposition (MID)
appeared together with the photodissociation. This is due
to the metastable decay of the aniline ion generated by
three-photon absorption in the source. This MID contamination was eliminated as follows. The PD laser pulse
repetition rate was reduced to 25 Hz. The MIKE signals for
two successive MPI pulses were recorded, one with and the
other without the PD pulse. Then, the latter was subtracted
from the former.
RESULTS AND DISCUSSION
In the MPI mass spectra of n-butylbenzene, bromobenzene,
iodobenzene and aniline at 266 nm, several fragment ions
appeared (Fig. 2). The ionization energies5,25 (IE) of all the
investigated molecules, listed in Table 1, are smaller than
2hn1, where n1 is the frequency of the MPI laser. The
fragment ions might be produced via both MPID and MPDI
processes, as mentioned earlier. Appearance energies (AE)
of most of the fragment ions from the molecules are higher
than 2hn1. This means that absorption of more than two
photons also occurred under the present MPI conditions.
When the molecular ions generated by MPI are selected by
the magnetic sector and dissociated by the absorption of PD
photon(s), as is the case in the present experiment,
complications due to MPDI can be eliminated. When a
high intensity laser is used for photodissociation, the
neutrals generated by PD may undergo MPI, resulting in
Copyright # 1999 John Wiley & Sons, Ltd.
INTERNAL ENERGY CONTENT IN TWO-PHOTON IONIZATION
1517
added to the ion internal energies, as described previously.5,9,13,14
n-Butylbenzene
MIKE spectra of photodissociation products at 355 and
266 nm of the n-butylbenzene molecular ion generated by
MPI, namely MPI-PD-MIKES, are shown in Fig. 3. The
profile was resolved to m/z 91 and 92 components assuming
a Gaussian profile for each, and the 91/92 ratio was
determined using the areas of the resolved peaks. Based on
the ion-optical calculations,26 the instrumental discrimination for these peaks were assumed to be the same. The ratios
of 91:92 are 1.2 and 2.9 for the PD experiments at 355 and
266 nm, respectively. The internal energy of the molecular
ion undergoing photodissociation was estimated from the
energy dependence of the branching ratio obtained previously using PEPICO.5 The branching ratios of 1.2 and 2.9
correspond to molecular ion internal energies of 4.15 and
5.24 eV, respectively.
Assuming that the molecular ion is generated by twophoton ionization at 0 K, the energy available as its internal
energy is given by
E0 avail ˆ 2h1 ÿ IE
1†
Since the electron ejected in the ionization carries some
kinetic energy, KEe, the internal energy of the molecular ion
becomes
E0 ˆ 2h1 ÿ IE ÿ KEe
Figure 2. MPI mass spectra at 266 nm of (a) n-butylbenzene, (b)
bromobenzene, (c) iodobenzene and (d) aniline.
another type of MPDI signal. This will be negligible
compared with the PD signal itself, especially when the PD
laser intensity is not excessively high. Then, the remaining
problem with regards to the energetics and dynamics of the
dissociation is the number of photons absorbed in the
generation of molecular ions which arrive at the PD region
20–30 ms after their formation. One obvious technique to
increase the relative abundance of the molecular ions
formed by the absorption of two photons is to use a low MPI
laser intensity. This technique was utilized in the present
work only as a test that the change in the MPI laser intensity
did not affect the kinetic data. According to the previous
kinetic studies of n-butylbenzene,5,12,21 bromobenzene6,7,13,22,23 and iodobenzene8,14,24 molecular ions, those
ions generated by the absorption of 2hn1 and excited by hn1
dissociate with rate constants of 108 sÿ1 or larger. Similar
results were observed in this work also. Since these ions will
dissociate rapidly in the ion source, it can be assumed that
the molecular ion beam arriving at the PD region consists of
ions generated by two-photon ionization only. For convenience of explanation, the thermal vibrational energies were
Copyright # 1999 John Wiley & Sons, Ltd.
2†
In this work 2hn1 is fixed at 9.32 eV. At room temperature,
the internal energy would be higher than the above due to
the thermal energy. The difference (Eth) may be approximated as the thermal energy of the corresponding neutral. Its
average value can be estimated with the vibrational
frequencies. The average thermal energies evaluated for nbutylbenzene and other molecules studied in this work are
summarized in Table 1. Then, the internal energy content of
the molecular ion after the absorption of hn2 becomes:
E ˆ E0 ‡ h2 ‡ Eth
3†
Once E* has been determined from the 91/92 branching
ratio, E0 can be estimated with the knowledge of hn2 and Eth.
Average values of E0 thus estimated are 0.46 and 0.38 eV
for the PD experiments at 355 and 266 nm, respectively.
The small difference reflects the error limit of the present
experiment, which has been quoted as 0.1 eV.15 The
internal energies estimated in various forms are summarized
in Table 1. E0 of 0.42 eV, the average of the above two, is
64% of the available energy (E0avail, 0.66 eV).
In the above analysis, we assumed that the internal energy
of the molecular ion generated by MPI has two origins, one
from the thermal energy of the neutral and the other from the
ionization process, and that the two can be treated
separately. To test this idea, we measured the 91/92 ratio
in 355 nm photodissociation and estimated the corresponding internal energy (E*) at various MPI source temperatures, 25–200 °C. E* values thus obtained are plotted as a
function of the average Eth in Fig. 4. E* increases with Eth
with a slope of 1.08. The linear interpolation results in the
intercept of 3.94 eV at 0 K. Subtracting hn2 from this
intercept results in a E0 value of 0.45 eV, in agreement with
that from the room temperature experiment. This supports
Rapid Commun. Mass Spectrom. 13, 1515–1521 (1999)
1518
INTERNAL ENERGY CONTENT IN TWO-PHOTON IONIZATION
Table 1. Energetics and measured branching ratios and rate constantsa
Molecular ion
n-Butylbenzene‡
n-Butylbenzene‡
Bromobenzene ‡
Iodobenzene‡
Aniline‡
PD-
(nm)
91/92 ratio
kc
(sÿ1)
E*b
(eV)
IEc
(eV)
E0availd
(eV)
Ethe
(eV)
E0f
(eV)
% E0g
355
266
266
355
266
1.2 0.1
2.9 0.1
—
—
—
—
—
(3.1 0.3) 108
(1.8 0.3) 108
(4 2) 105
4.15 0.08
5.24 0.06
4.83 0.05
4.00 0.05
5.5 0.1
8.66
8.66
8.98
8.69
7.72
0.66
0.66
0.34
0.63
1.60
0.20
0.20
0.04
0.04
0.03
0.46 0.08
0.38 0.06
0.13 0.05
0.47 0.05
0.8 0.1
70 12
58 9
38 15
75 8
50 6
a
See text for the definitions of the internal energies in various forms (Eqns (1–3)). Errors quoted were estimated from several duplicate measurements at the 95%
confidence limit except those for aniline‡ . See text for aniline‡ .
b
Internal energies of molecular ions undergoing dissociation determined by the kinetic data.
c
Ionization energy in Ref. 3 for n-butylbenzene and Ref. 25 for others.
d
The energy available as the molecular ion internal energy in the two-photon ionization, 2hn1 ÿ IE.
e
Average (n-butylbenzene) and most probable (others) thermal internal energy calculated with vibrational frequencies in Refs 12–14, and 30 for n-butylbenzene,
bromobenzene, iodobenzene and aniline, respectively.
f
Internal energies of molecular ions generated by two-photon ionization estimated with the kinetic data.
g 0 0
E /E avail in %.
our separate treatment of the thermal internal energy. Also
of importance is the fact that the thermal internal energy
participates actively in the dissociation process.
Bromobenzene
The MPI-PD-MIKE spectrum of bromobenzene was
obtained with the 355 nm PD laser. Loss of bromine was
the only dissociation channel. A time-resolved PD-MIKE
spectrum is shown in Fig. 5(a). The spectrum was obtained
under the condition that photodissociation occurred in the
presence of the electric field of 2 kV/cm. The asymmetric
Figure 3. MPI-PD-MIKE spectra of the n-butylbenzene molecular ion
with PD at (a) 355 and (b) 266 nm. Dotted lines are the m/z 91 and 92
components resolved assuming a Gaussian profile for each.
Rapid Commun. Mass Spectrom. 13, 1515–1521 (1999)
profile indicates that the dissociation occurs on a nanosecond time scale. The method to determine the PD rate
constant or its distribution by analyzing a time-resolved PDMIKE profile has been previously described in detail.11,13
The most probable rate constant (kc) determined from the
time-resolved profile (Fig. 5(a)) is 3.2 108 sÿ1. The
profile calculated from the best-fit rate constant distribution
is also shown in the figure. Several duplicate experiments
resulted in an average kc of 3.1 0.3 108 sÿ1. In the
photodissociation at 266 nm, the MIKE profile appeared
symmetric indicating that the rate constant was larger than
8 108 sÿ1, which is the upper limit that can be detected by
the present technique.
Rate-energy dependence for the Br loss of the bromobenzene ion was determined by several different techniques.6,7,13,22,23 All the experimental data obtained over the
time range of nanoseconds to milliseconds could be fit to a
single theoretical model calculation. The data were
summarized in our previous report.13 Based on the data,
the ion internal energy corresponding to the rate constant
measured is 4.83 eV. E0 estimated with this value is 0.13 eV
which is 38 % of the available energy (0.34 eV) in the twophoton ionization (2PI) process (see Table 1).
Figure 4. Internal energy (E*) of the n-butylbenzene molecular ion
photoexcited at 355 nm versus average thermal internal energy. Filled
circles represent experimental data. The linear regression of the
experimental data results in the solid line with the slope and intercept
of 1.08 0.14 and 3.94 0.05, respectively.
Copyright # 1999 John Wiley & Sons, Ltd.
INTERNAL ENERGY CONTENT IN TWO-PHOTON IONIZATION
Figure 5. Time-resolved MPI-PD-MIKE spectra of molecular ions
of (a) bromobenzene with 266 nm PD laser and an electric field of
2kV/cm, (b) iodobenzene with 355 nm PD laser and an electric field of
2kV/cm and (c) aniline with 266 nm PD laser and an electric field
of 0.5kV/cm. Experimental and calculated results are shown as filled
circles and solid curves, respectively.
Iodobenzene
Dietz et al. reported that no ions were observed when a
nanosecond laser, in the range 259–272 nm, irradiated an
iodobenzene sample.27 This is due to dissociation in the S1
state which occurs with a lifetime less than 5ps, as reported
by Wilkerson and Reilly.28 In this work, however, the
molecular and fragment ions were observed as shown in the
MPI mass spectrum. (Fig. 1(c)) The main difference
between the previous27 and present MPI studies is in the
MPI laser intensity. The fact that a very high MPI laser
intensity can be used and that only those ions which are
Copyright # 1999 John Wiley & Sons, Ltd.
1519
generated by the absorption of two MPI photons can be
selected for photodissociation study, with the present
technique, are particularly advantageous in this regard.
Loss of iodine was the only dissociation channel found in
the MPI-PD-MIKE spectrum at 355 nm. A time-resolved
MPI-PD-MIKE spectrum profile obtained with an electric
field of 2 kV/cm is shown in Fig. 5(b). The most probable
rate constant determined by analyzing the profile is
1.8 108 sÿ1. The rate-energy dependence of this dissociation is also well established and was summarized in our
previous report.14 The ion internal energy corresponding to
the measured rate constant here can be read from the data,
which is 4.00 eV. E0 estimated using Eqn. (3) is 0.47 eV
which is 75% of the available energy (0.63 eV) in the 2PI
process (see Table 1).
To summarize the discussion made so far, the internal
energies of the molecular ions generated by MPI and excited
by a PD laser have been estimated from the experimental
data. Thus the average internal energies (E0) deposited to
the molecular ions in the ionization processes have been
estimated assuming two-photon ionization (2PI) and onephoton dissociation (1PD). E0 values thus estimated were
38–75% of the available internal energies in 2PI. This result
is in agreement with our assumption of 2PI and 1PD.
An accurate knowledge of the internal energy distribution
for molecular ions generated by MPI can be obtained by
recording the photoelectron spectrum. MPI-photoelectron
spectra have been reported for several substituted benzenes.
The MPI-photoelectron spectra show a few common
features.29,30 One is the ‘Dn = 0 propensity rule’ and another
is that unless 000 …S1
S0 † transition is involved, the internal
energy distribution continues up to the available energy.
The present result that E0 values are around 50% of the
available energies in 2PI is in general agreement with the
conclusion from the MPI-photoelectron studies. In the
multiphoton ionization dissociation study of 2,4-hexadiyne
by Szaflarski and co-workers,31 it was reported that the
internal energy of the molecular ion undergoing dissociation
increased linearly with the photon energy. If the MPID
mechanism is responsible for the reaction observed, the
above result means that the internal energy content of the
molecular ion generated by two-photon ionization remains
the same regardless of the laser wavelength. The present
result is not in general agreement with the above, even
though the approaches taken in the two studies are different.
It is to be pointed out that kinetic properties of the systems
chosen in this work are better established than that of the
2,4-hexadiyne molecular ion. In our previous PD-MIKE
studies of ion dissociation, efforts were made to obtain
kinetic data with an internal energy accuracy of 0.1 eV.
Taking 50% of the available energy in 2PI as the average ion
internal energy is a decent estimate when the difference
between the ionization energy and 2hn1 is not large as are
the cases for the systems investigated above. As a check of
the 50% rule, we also investigated the MPI-PD of aniline
(IE = 7.72 eV25) for which the difference between IE and
2hn1 is large (1.60 eV).
Aniline
In the MPI-PD of aniline with the 266 nm PD laser, HNC
loss was the only observed channel. A time-resolved PDMIKE spectrum obtained with the electric field of 0.5 kV/
cm is shown in Fig. 5(c) The spectrum consists of two
components, an asymmetrically broadened peak and a sharp
Rapid Commun. Mass Spectrom. 13, 1515–1521 (1999)
1520
INTERNAL ENERGY CONTENT IN TWO-PHOTON IONIZATION
one. The broad and sharp peaks are the results of
dissociations inside and outside the field region, respectively. The high abundance of the sharp component means
that the dissociation occurred very slowly. Even though the
reaction was a little too slow for its rate constant to be
determined reliably with the present technique, an attempt
was made to analyze the data. It was found that the timeresolved profile could be fit with a distribution of rate
constant with the most probable value (kc) in the range of 2–
6 105 sÿ1. The PD-MIKE profile calculated with a rate
constant distribution (kc = 4 105 sÿ1) is also shown in Fig.
5(c). The rate constant measurement for the same reaction
using MPI at 266 nm was reported by Proch et al.19
Experimental data were analyzed assuming a single rate
constant with 2 106 sÿ1 resulting in the best fit. Our data
fits with the same rate constant. Considering that the
available energy is very large in this case (1.60 eV), it is
likely that the present approach of using a broad rate
constant distribution is better than the use of a single rate
constant. We do not claim high accuracy with the present
results, however, as the rate is beyond the reliable range for
the technique. The rate-energy dependence for the same
reaction was obtained using PEPICO.9 The internal energy
of the molecular ions undergoing photodissociation estimated by comparing the present and PEPICO data is 5.4–
5.6 eV. This results in an E0 value of 0.7–0.9 eV, which is
50% of the available energy (1.60 eV) in 2PI (see Table
1). The above estimate is likely to be an upper limit because
the dissociation of aniline ions with much lower internal
energy would not be detected at all by the present technique.
It is gratifying, however, to note that the 50% rule applies
for a system with a large available energy in MPI even
though only approximately.
The slow decay of the aniline molecular ion after the
absorption of a 266 nm photon is useful to explain the
observation of the metastable ion decomposition (MID) of
the aniline molecular ions generated by MPI. If these ions
are generated by the absorption of two 266 nm photons and
excited by an additional 266 nm photon, the most probable
rate constant for their dissociation would be around 4 105
sÿ1, as determined by MPI-PD. Then, observation of their
dissociation after a time delay of around 20ms is not
surprising at all. If the rate constant is larger than the above,
say 2 106 sÿ1 as obtained by the single rate constant
analysis, we would expect hardly any dissociation after the
experimentally defined time delay. A simple kinetic
analysis with the rate constant of 4 105 sÿ1 shows that
only a fraction (0.03%) of molecular ions associated with
three-photon absorption remains after a time delay of 20 ms.
If these ions absorb one more 266 nm photon in the
photodissociation step, fast dissociation is expected. The
fact that such a fast dissociation component was not
detected in the time-resolved PD-MIKE spectrum indicates
that their contribution to MPI-PD is negligible, in agreement
with the above kinetic analysis.
CONCLUSIONS
A technique based on MIKES has been devised to
investigate the photodissociation kinetics of molecular ions
generated by MPI on a nanosecond time scale. From the
measurement of the branching ratio or rate constant for the
photodissociation of the ionic systems with well-established
kinetic information, the ion internal energies were estimated. The results show that only those molecular ions
Rapid Commun. Mass Spectrom. 13, 1515–1521 (1999)
generated by two-photon ionization in the source make
important contributions to the photodissociation signals.
Spatial (277 cm) and temporal ( 20 ms) separation between
ionization and dissociation are responsible for the above
observations, which is an important aspect of the present
technique.
The average internal energies of the molecular ions
generated by two-photon ionization were around 50% of the
available energies (2hn1 ÿ IE). This result is in agreement
with the usual patterns of the reported MPI-photoelectron
spectra. In addition, it has been demonstrated that the
thermal internal energy of the molecule may be assumed to
be conserved in the ionization process. Hence, adding
thermal internal energy to half of the available energy
provides a reasonable estimate for the internal energy of
molecular ions generated by MPI. This estimate will be
especially reliable for most of the substituted benzene ions
since their ionization energies are usually close to 9.3 eV,
which corresponds to the two-photon energy at 266 nm. For
these systems, the MPI-PD technique reported in this work
would be useful to study their dissociation dynamics at welldefined internal energy.
Acknowledgements
This work was supported financially by CRI, the Ministry of Science
and Technology, Republic of Korea.
REFERENCES
1. J. Jortner, R. D. Levine and B. Pullman (Eds), Mode Selective
Chemistry, Kluwer Academic Publishers, Dordrecht (1991); J. C.
Whitehead (Ed.), Selectivity in Chemical Reactions, Kluwer
Academic Publishers, Dordrecht (1988).
2. P. J. Robinson and K. A. Holbrook, Unimolecular Reactions, John
Wiley, New York (1972).
3. T. Baer, Adv. Chem. Phys. 64, 111 (1986).
4. J. W. Keister, T. Baer, R. Thissen, C. Alcaraz, O. Dutuit, H. Audier
and V. Troude, J. Phys. Chem. A 102, 1090 (1998).
5. T. Baer, O. Dutuit, H. Mestdagh and C. Rolando, J. Phys. Chem.
92, 5674 (1988).
6. T. Baer and R. Kury, Chem. Phys. Lett. 92, 659 (1982).
7. H. M. Rosenstock, R. Stockbauer and A. C. Parr, J. Chem. Phys.
73, 773 (1980).
8. J. Dannacher, H. M. Rosenstock, R. Buff, A. C. Parr, R. L.
Stockbauer, R. L. Bombach and J. P. Stadelmann, Chem. Phys. 75,
23 (1983).
9. T. Baer and T. E. Carney, J. Chem. Phys. 76, 1304 (1982).
10. R. C. Dunbar, J. Phys. Chem. 91, 2801 (1987); R. C. Dunbar and
C. Lifshitz, J. Chem. Phys. 94, 3542 (1991); F. Huang and R. C.
Dunbar, Int. J. Mass Spectrom. Ion Phys. 109, 151 (1991).
11. J. C. Choe and M. S. Kim, J. Phys. Chem. 95, 50 (1991); W. G.
Hwang, J. H. Moon, J. C. Choe and M. S. Kim, J. Phys. Chem. A
102, 7512 (1998).
12. S. T. Oh, J. C. Choe and M. S. Kim, J. Phys. Chem. 100, 13367
(1996).
13. S. H. Lim, J. C. Choe and M. S. Kim, J. Phys. Chem. A 102, 7375
(1998).
14. Y. H. Yim and M. S. Kim, J. Phys. Chem. 97, 12122 (1993).
15. Y. H. Yim and M. S. Kim, J. Phys. Chem. 98, 5201 (1994).
16. E. W. Schlag and H. J. Neusser, Acc. Chem. Res. 16, 355 (1983);
H. Kühlewind, H. J. Neusser and E. W. Schlag, J. Phys. Chem. 88,
6104 (1984); H. J. Neusser, J. Phys. Chem. 93, 3897 (1989).
17. R. J. Stanley, M. Cook and A. W. Castleman, Jr., J. Phys. Chem.
94, 3668 (1990).
18. J. L. Durant, D. M. Rider, S. L. Anderson, F. D. Proch and R. N.
Zare, J. Chem. Phys. 80, 1817 (1984).
19. D. Proch, D. M. Rider and R. N. Zare, Chem. Phys. Lett. 81, 430
(1981).
20. D. A. Gobeli, J. J. Yang and M. A. El-Sayed, Chem. Rev. 85, 529
(1985); J. J. Yang, D. A. Gobeli, R. S. Pandolfi and M. A. ElSayed, J. Phys. Chem. 87, 2255 (1983).
21. E. S. Mukhtar, I. W. Griffiths, F. M. Harris and J. H. Beynon, Int. J.
Copyright # 1999 John Wiley & Sons, Ltd.
INTERNAL ENERGY CONTENT IN TWO-PHOTON IONIZATION
22.
23.
24.
25.
26.
27.
Mass Spectrom. Ion Phys. 37, 159 (1981); S. Nacson and A. G.
Harrison, Int. J. Mass Spectrom. Ion Processes 63, 85 (1985); M.
J. Welch, D. J. Pereles and E. White, Org Mass Spectrom. 20, 425
(1985); J. H. Chen, J. D. Hays and R. C. Dunbar, J. Phys. Chem.
88, 4759 (1984).
S. T. Pratt and W. A. Chupka, Chem. Phys. 62, 153 (1981).
Y. Malinovich, R. Arakawa, G. Haase and C. Lifshitz, J. Phys.
Chem. 89, 2253 (1985).
Y. Malinovich and C. Lifshitz, J. Phys. Chem. 90, 2200 (1986).
S. G. Lias, J. E. Bartmess, J. F. Liebman, J. L. Holmes, R. D. Levin
and W. G. J. Phys. Chem. Ref. Data 17, Suppl. 1 (1988).
I. C. Yeh and M. S. Kim, Rapid Commun. Mass Spectrom. 6, 115
(1992).
T. G. Dietz, M. A. Duncan, M. G. Liverman and R. E. Smalley, J.
Chem. Phys. 73, 4816 (1980).
Copyright # 1999 John Wiley & Sons, Ltd.
1521
28. C. W. Wilkerson, Jr. and J. P. Reilly, Anal. Chem. 62, 1804 (1990).
29. J. P. Reilly, Isr. J. Chem. 24, 266 (1984); J. T. Meek, S. R. Long
and J. P. Reilly, J. Phys. Chem. 86, 2809 (1982); S. R. Long, J. T.
Meek and J. P. Reilly, J. Chem. Phys. 79, 3206 (1983); J. T. Meek,
E. Sekreta, W. Wilson, K. S. Viswanathan and J. P. Reilly, J.
Chem. Phys. 82, 1741 (1985); K. Walter, K. Scherm and U. Boesl,
J. Phys. Chem. 95, 1188 (1991); K. Kimura and M. Takahashi, J.
Chem. Phys. 97, 2920 (1992).
30. X. Song, M. Yang, E. R. Davidson and J. P. Reilly, J. Chem. Phys.
99, 3224 (1993).
31. D. M. Szaflarski, E. L. Chronister and M. A. El-Sayed, J. Phys.
Chem. 91, 3259 (1987).
Rapid Commun. Mass Spectrom. 13, 1515–1521 (1999)