State-to-state photodissociation studies by
VUV-photodissociation-pump and
VUV-photoionization-probe method
Cheuk-Yiu Ng
Department of Chemistry
University of California, Davis
Photo dissociation in Astrochemistry
Leiden Observatory Workshop
(Feb. 3-5, 2015)
Neutral Photodissociation processes in the VUV
range were labeled as “dark reactions”
Most neutral photodissociation processes have not been
explored because of the lack of intense tunable VUV
light sources. Can we now take on this challenge?
Improvement in VUV laser source:
•
•
Synchrotron VUV: Resolution = 1cm-1 and intensity =
109 - 1010 photons/s
VUV laser by 4-wave mixing: Resolution = 0.1 cm-1
and intensity = 1012-1014 photons/pulse
Vacuum Ultraviolet Laser Tunable range (7.0-19.0 eV)
Four-wave sum and differencefrequency mixings in rare gases or
metal vapors: high efficiency
The Simulation of VUV Laser Separation
from Fundamentals by Convex Lens
Rcell (um)
RSlit (um)
Y(mm)
()
Visible (2)
57.8
443
-17.9
3.4
UV (1)
17.3
512
-19.6
3.7
VUV (2 1 -2)
16.6
119
-25.0
4.7
12cm
30cm
8mm
Gas Cell

Y
MgF2 Bi-Convex Lens
Without using defraction grating:
Achievable tunable VUV
Intensities upto : 1012-1014/pulse
The surface of Slit
Images and simulation were done by optical software CODE V
VUV laser velocity-mapped ion- and electronimaging appartatus
Tunable VUV
laser radiation
Molecular
beam
Imaging MCP
Photodissociation laser 193 nm
Imaging TOF chamber
State-to-state photodissociation Study
State-to-state photodissociation studies by
• VUV laser photodissociation pump
• VUV laser photoionization probe
Goals: To apply on photodissociation
Atmospheric gases CO, N2, and CO2 etc.
CO is the second most abundant molecular species after H2 in the interstellar
medium. Thus, VUV photodissociation study of CO is very important to
understand the properties of the interstellar medium, planet formation, and Catom and O-atom isotope fractionation.
CO photodissociation in the VUV region is still largely unknown.
C(3P) + O(1D)
C(1D) + O(3P)
C(3P) + O(3P)
hv
M. Eidelsberg, F. Launay, K. Ito, T. Matsui, P. C. Hinnen, E. Reinhold, W. Ubachs, and K. P. Huber, J. Chem.
Phys., 121 (1), 292 (2004).
Solar VUV Irradiance in the range shorter than 200 nm
Irradiance (photons/s/cm3)
Lyman β
1012
1011
1010
109
108
0
50
100
150
200
Wavelength (nm)
Relevant to COSS: 91.17-111.78 nm (11.09-13.60 eV)
Experimental plan for VUV photodissociation-pump
and VUV photoionization-probe
CO(X1)  C(3P) + O(3P)
 C(1D) + O(3P)
 C(3P) + O(1D)
E = 11.09 eV
E = 12.37 eV
E = 13.08 eV
By tuning ω2 in the range of 400-900 nm with ω1 fix at the nonlinear
medium (Kr or Xe):
•The difference-frequency (2ω1-ω2) and sum-frequency (2ω1+ω2) can
be generated in the respective ranges of 6.9-11.5 and 11.3-16.0 eV.
•Difference-frequencies for photodissociation excitation
•Sum frequencies for photoionization sampling
Development of the VUV laser velocity-mapped
imaging photoion (VMI-PI) apparatus
CO + VUV  C(3P) + O(3P)
C(1D) + O(3P)
C(3P) + O(1D)
11.05 eV
12.31 eV
13.02 eV
C(3P) + VUV  C+ + e-
C(1D) + VUV  C+ + e-
We found that photodissociation and photoionization
can be accomplished with the same laser pulse!
Branching Ratio Measurements
(a) (b): R(0) line of (4pσ)1Σ+(v'=3) at 109484.7 cm-1
(c) (d): R(0) line of (4sσ)1Σ+(v'=4) at 109452.5 cm-1
Branching Ratio
measurements :
25 identified
predissociative
vibronic bands
Above dissociation
energy of CO
R. Visser, E. F. van Dishoeck, and J. H. Black, Astron. Astrophys. 503 (2), 323 (2009).
Branching Ratio Measurements for CO
Dissociation into the
channel C(1D) + O(3P)
The branching ratio into the spin-forbidden channel strongly depends
on the vibronic state of CO excited by the VUV photon.
Rotational dependence
Dissociation into the
channel C(1D)+O(3P)
Strong rotational
dependence
PFI-PI and PIE bands of O(3P0:3P1:3P2) formed by
photodissociation
at 193
and212.5
PFI-PI + PIE Specrta of
of OSO
atom2 [SO
+ 193.3nm
SO + O( nm
P )]
3

50
40

30
50
25
40
109678.558
109609.831
0

60
109400
50
40
109300
30
25
109200
3
109500
P2
4
109600
o
3
o
3
O ( S )nd D  P2,1,0
109836.967
8
n=34
25
+
I(O+) (arb. Units)
I(O )
2
P0
30
3
25
P1
20
3
15
P2
16
3
2,1,0
20
4
25
2
0
109700
109800
109900
VUV, cm
-1
110000
SO2 + h(193.3 and 212.5 nm) → SO(v) + O(3P2)
110100
Total kinetic energy release spectrum for SO2
photodissociation at 193 and 212.5 nm obtained
Rydberg tagging of O(3P2)
=3
P(Ec.m.) (arbit. units)
1.5
1.0
2
1
0
=0
0.5
0.0
0.2
0.4
Ec.m. (eV)
0.6
0.8
C(3P0,1,2) Fine Structure Distribution by VUV-UV (1+1’)
state-selective photoionization
Ionization Continuum
UV or VIS
2s22p4s (3P2)
2s22p4s (3P1)
2s22p4s (3P0)
VUV
3P
2
3P
1
3P
0
C(3P0,1,2) Fine Structure Distribution VUV-UV (1 + 1’) detection
State-selective VUV-(1+1’) photoionization
Fine structure distributions (in %) for C(3P0, 3P1, and 3P2) formed by
VUV photodissociation of CO excited in the N = 1 rotational levels
of the (4sσ)1Σ+(v = 4), (4pσ)1Σ+(v = 3), and (4pπ)1Π(v = 3) states
Predissociative
CO states
Fine structure distribution in %
via common state:
C*[2s22p4s (3P1)]
via Common state :
C*[2s22p3d (3D1)]
3P
0
3P
1
3P
2
3P
0
3P
1
3P
2
(4sσ)1Σ+(v=4)
69 ± 2
10 ± 2
21 ± 2
67 ± 4
8±3
25 ± 3
(4pσ)1Σ+(v=3)
54 ± 2
24 ± 2
22 ± 2
51 ± 4
23±3
26 ± 2
(4pπ)1Π(v=3)
28 ± 4
40 ± 4
32 ± 5
30 ± 9
33±12
37 ± 2
C(3P2) + O(1D) -------- (BR-III)*(F2)
C(3P) + O(1D)
C(3P1) + O(1D) -------- (BR-II)*(F1)
C(3P0) + O(1D) -------- (BR-I)*(F0)
CO + VUV
C(3P2) + O(3P) -------- [1-(BR-III)]*(F2)
C(3P) + O(3P)
C(3P1) + O(3P) -------- [1-(BR-II)]*(F1)
C(3P0) + O(3P) -------- [1-(BR-I)]*(F0)
BR-I = [C(3P0) + O(1D)] / { [C(3P0) + O(3P)] + [C(3P0) + O(1D)] }
BR-II = [C(3P1) + O(1D)] / { [C(3P1) + O(3P)] + [C(3P1) + O(1D)] }
BR-III = [C(3P2) + O(1D)] / { [C(3P2) + O(3P)] + [C(3P2) + O(1D)] }
F0 = [C(3P0)] / {[C(3P0)] + [C(3P1)] + [C(3P2)]}
F1 = [C(3P1)] / {[C(3P0)] + [C(3P1)] + [C(3P2)]}
F2 = [C(3P2)] / {[C(3P0)] + [C(3P1)] + [C(3P2)]}
BR-I
BR-II
BR-III
(4sσ)1Σ+(v=4)
0.62±0.03
0.09±0.01
0.08±0.01
(4pσ)1Σ+(v=3)
0.37±0.02
0.03±0.02
0.12±0.01
(4pπ)1Π(v=3)
0.20±0.01
0.59±0.03
0.13±0.01
Correlated fine structure distribution of the channel C(3P0,1,2) + O(1D2) [O(3PJ)]
C(3P2)+O(1D)
C(3P1)+O(1D)
C(3P0)+O(1D)
C(3P2)+O(3PJ) C(3P1)+O(3PJ) C(3P0)+O(3PJ)
(4sσ)1Σ+(v=4)
1.7±0.3
0.9±0.2
42.4±3.0
19.3±2.0
9.1±1.9
26.6±2.3
(4pσ)1Σ+(v=3)
2.7±0.4
0.7±0.4
19.7±1.6
19.3±1.9
23.3±2.3
34.3±2.2
(4pπ)1Π(v=3)
4.3±1.1
23.4±3.4
5.5±0.9
27.7±4.8
16.6±2.7
22.5±3.3
VUV Photodissociation of CO2
CO2
Mars
Venus
Early earth’s atmosphere
Carrier of O2
VUV-VUV-VMI-PI apparatus
Photoproduct channels for VUV photodissociation
of CO2
CO2 + hv → CO(X 1Σ+) + O(3P)
CO2 + hv → CO(X 1Σ+) + O(1D)
CO2 + hv → CO(X 1Σ+) + O(1S)
CO2 + hv → CO(a 3Π) + O(3P)
CO2 + hv → CO(a 3Π) + O(1D)
CO2 + hv → CO(a′ 3Σ+) + O(3P)
CO2 + hv → CO(d 3∆) + O(3P)
CO2 + hv → CO(e 3Σ-) + O(3P)
CO2 + hv → CO(A1Π) + O(3P)
CO2 + hv → CO(I 1Σ-) + O(3P)
CO2 + hv → CO(D 1∆) + O(3P)
hv > 5.45 eV
hv > 7.42 eV
hv > 9.64 eV
hv > 11.46 eV
hv > 13.43 eV
hv > 12.31 eV
hv > 12.97 eV
hv > 13.35 eV
hv > 13.48 eV
hv > 13.45 eV
hv > 13.56 eV
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
Comparison of absorption and
O(3P2) photofragment spectra of CO2
The detection of O atoms
VUV-Visible
photoionization
VUV autoionization
Z. Lu, Y. C. Chang, H. Gao, Y. Benitez, Y. Song, C. Y. Ng and W. M. Jackson, Journal of Chemical
Physics, In press (2014).
Decoding the photochemistry of CO2
hv
The fine structure branching ratio of
CO(a3Π) + O(3PJ) and CO(X1Σ+) + O(3PJ) channels
at CO2 4s Rydberg state
The VMI-PI images and corresponding KER spectra for
the CO(X1Σ+) + O(1S) channel recorded at (a)12.125 eV,
(b) 12.145 eV, and (c)12.150 eV.
Vibrational population
Plot of β parameters as a
function of v
The VMI-PI images and corresponding KER spectra for
the CO(X1Σ+) + O(1D) channel recorded at (a)12.125 eV,
(b) 12.145 eV, and (c)12.150 eV.
CO2 photodissociation: angular distribution of the CO(ν) +
O(3P2,1,0) [O(1D), and O(1S)] photofragment channels
CO (1Σ+) + O(3P2)
CO (1Σ+) + O (1D)
CO (1Σ+) + O (1S)
Calculated Excited CO2 potential energy surfaces
Singlet potential energy surfaces calculated at
MRCI level of theory
• CO(X1Σ+) + O (1S) channel: exclusively via 4 1Aʹ PES
• CO(X1Σ+) + O (1D) channel: via 3 1Aʹ PES from conical
intersection between 3 1Aʹ and 4 1Aʹ PES at ~3.5 bohr
Comparison of CO2 absorption spectrum with the C(3P2) and
O(1S) PHOFEX spectra
L. Archer et al. Journal of
Quantitative Spectroscopy
and Radiative Transfer
117, 88 (2013)
VUV2-Vis photoionization
[2s22p3d (3D°3)]
VUV2 autoionization
[2s22p3(2P°)3s (1P°1)]
C is an exit channel in CO2 photodissociation
C(3P2) photofragment
excitation spectrum
hv
O
Energy (eV)
C
Roaming
Pathway 2
C
C
O
O
O
OC … O
O
C(3P) + O2(X3Σg-)
O
O
O
C
O
(11.44 eV)
C
O
O
C
1A
1
hv
(7.13 eV)
Singlet
Pathway 1
(6.03 eV)
O
C
O
1Σ+
CO2(X1Σg+)
D. Y. Hwang and A. M. Mebel, Chemical Physics 256, 169 (2000)
S. Y. Grebenshchikov, The Journal of Chemical Physics 138, 224106
(2013)
C+ ion TOF spectra
TOF spectrum at the
CO2 (3p1Πu103) Rydberg state
CO2 + hν(VUV1) → C(3PJ) + O2(X3Σg-)
C(3PJ) + hν(VUV2) → C+ + e-
The C+ ion signal relates to both the photodissociation
(VUV1) and photoionization (VUV2) laser radiations
C(3P2) velocity-map ion images
Threshold of the C(3P) + O2(X3Σg-) channel
VUV photodissociation of N2
Photodissociation of N2:
N2 + hv1 → N(4S) + N(4S)
N2 + hv1 → N(4S) + N(2D)
N2 + hv1 → N(4S) + N(2P)
N2 + hv1 → N(2D) + N(2D)
VUV2
hv ≥ 9.759 eV
hv ≥ 12.139 eV
hv≥ 13.339 eV
hv ≥ 14.529 eV
Relative intensity (arb. unit)
VUV1
1
+
b' u (v'=12)
4
4
5
0
0.0
2
N( S)+N( D)
10
2
N( S)+N( P)
0.5
1.0
1.5
2.0
TKER (eV)
N(4S) + hv2 → N+ + e- or
N(2D) + hv2 → N+ + e-
2.5
Branching ratios for the spin-forbidden N(4S) + N(2D) and N(4S) + N(2D)
channels and the spin-allowed N(2D) + N(2D) channel from N2 valence and
Rydberg states with 1Πu symmetry. The upward arrows indicate the
thresholds of the N(4S) + N(2P) and N(2D) + N(2D) channels
Branching ratios for the spin-forbidden N(4S) + N(2D) and N(4S) + N(2D)
channels and the spin-allowed N(2D) + N(2D) channel from N2 valence and
Rydberg states with 1Σu+ symmetry. The upward blue arrows indicate the
threshold of the N(4S) + N(2P) and N(2D) + N(2D) channels.
Greetings from Ng Group 2013
Thank you:
Greetings from Ng Group 2013
Thank you: