Transcript ISMS_2007_CHFBdynamics.ppt

Mode specific dynamics in the predissociated,
quasilinear B 1A' state of CHF probed by opticaloptical double resonance spectroscopy
CHONG TAO, C. MUKARAKATE AND S. A. REID,
Department of Chemistry, Marquette University, Milwaukee, WI 53233.
T. W. SCHMIDT AND S. H. KABLE,
School of Chemistry, University of Sydney, NSW 2006, Australia
62nd International Symposium on Molecular Spectroscopy, June 20th, 2007
Electronic structure of carbenes
• Carbenes contain a
divalent carbon and
feature low-lying singlet
and triplet states
• Most of the studies are
focused on the singlettriplet gap and transitions
between RT pair in visible
region.
C
C
S0
T1
S1
T1
S0
Singlet Carbene
C
S1
Photochemistry of halocarbenes
• To date, almost nothing is known about the ultraviolet
photochemistry of halocarbenes, especially the
photodissociation dynamics.
• Prof. Kable’s group studied the dissociation of CFBr and
CFCl in the A state via curve crossing to form CF+Br and
Cl. However, the curve crossing is not accessible in the
FC region in other small halocarbenes.*
• Recently, Prof. Dagdigian’s group explored the193nm
photodissociation of CFBr and CFCl into CF+Br and
CF+Cl. The molecule may initially be excited to a bent
Rydberg state and then dissociate through a linear
intermediate state. But, the exact dissociative surfaces
remain uncertain.#
* J. Guss, O. Votava, and S. Kable, JCP.,115,11118 (2001).
# S. Shin and P. Dagdigian, JCP., 126, 134302 (2007)
Potential energy surfaces of
CHF (calculated)
~31000cm-1
Using MOLPRO
and CASSCF
method with ccpVTZ basis set.
Optical-optical double resonance
spectroscopy: methodology
Pump laser only
Pump laser + Probe laser
0
2
4
6
8
Time in s
CHF was produced by
discharge of CH2F2 in a
jet beam
•
•
All B state levels are broadened by dissociation
The B state is quasi-linear, levels labeled by
(v1,v2ℓ,v3). Even v2 contains only even ℓ components.
Mode specificity: adding CF stretch
enhances dissociation rate
1
1
(0,3 ,0)
-1
 = 2.1 cm
 = 2.4 ps
32500
32600
(0,1 ,1)
-1
 = 8.3 cm
 = 0.6 ps
32700
32800
2-photon energy in cm
-1
32900
More OODR spectra
2
(0,4 ,1)
0
(0,4 ,1)
2
(0,6 ,0)
0
2
Pump (1,1,0)
Ka=1
r
Pump R0(1)
(0,6 ,0)
(1,2 ,1)
0
(1,2 ,1)
0
(1,4 ,0)
0
(1,0 ,2)
2
(1,4 ,0)
34400
34500
34600
34700
34800
34900
35000
35100
r
Pump R0(2)
36400
36500
36600
36700
2
(0,2 ,3)
34400
34500
34600
34700
34800
34900
35000
Pump (0,1,1)
Ka=1
35100
r
Pump R0(3)
34400
34500
34600
34700
34800
2-photon energy in cm
34900
-1
35000
35100
36400
36500
36600
2-photon energy in cm
-1
36700
CH vs. CF stretch: OODR spectra
3
• Contrary to
our initial
expectations,
CH stretching
excitation
does not
enhance
the dissociation
rate
(0,3 ,1)
-1
 = 16 cm
 = 0.3 ps
0
(1,2 ,0)
-1
 = 6.2 cm
 = 0.8 ps
0
(0,1 ,2)
-1
 = 43 cm
 = 0.1 ps
34000
34200
34400
35000
2-photon energy in cm
35200
-1
35400
Linewidths vs. Energy: 1
• Combination
states with bending
and CF stretch
excitation display
larger widths, which
tend to increase with
increasing n3
60
l
(0,v2 ,0)
l
(0,v2 ,1)
50
-1
l
Linewidth in cm
• The narrowest
linewidths are
associated with
“pure” bending
or CF stretching
states
(0,v2 ,2)
40
l
(0,v2 ,3)
30
20
10
0
0
1000 2000 3000 4000 5000 6000 7000
Energy above B state origin in cm
-1
Linewidths vs. Energy: 2
• Combination
states with CF
stretching
excitation display
larger widths
40
l
(0,v2 ,0)
l
(1,v2 ,0)
30
l
(1,v2 ,1)
-1
Linewidth in cm
• Combination
states with only
CH stretch
display similar
linewidths to the
pure bending
states
l
(1,v2 ,2)
20
10
0
0
1000 2000 3000 4000 5000 6000 7000
Energy above B state origin in cm
-1
Summary of findings
• All B state levels are predissociated, as evidenced by
broadening Lorentzian lineshapes, indicating some form
of fast nonradiative decay
• The decay is mode specific – in particular, a combination
of bending and CF stretching excitation enhances the
rate
• Combination states with only CH stretch and bending
excitation display similar linewidths to the pure bending
states (CH stretching excitation has no effect on rate)
• Within the uncertainty of our measurements, and over
the limited range of J levels accessed, there appears to
be no dependence of the linewidth on rotational level or
vibrational angular momentum (ℓ) component
Possible dissociation channel?
50000
45000
CH + F
40000
C(1D) + HF
Energy in cm-1
35000
B
30000
25000
CF + H
C(3P) + HF
20000
A
15000
MP2
10000
a
5000
0
CHF
X
MP4
B3LYP
Aug-cc-pVTZ
basis set
2
Possible elimination channel?
50000
45000
CH + F
40000
C(1D) + HF
Energy in cm-1
35000
B
30000
25000
CF + H
C(3P) + HF
20000
A
15000
MP2
10000
a
5000
0
CHF
X
MP4
B3LYP
Aug-cc-pVTZ
basis set
2
Acknowledgements
People:
Calvin Mukrakate
Mihaela Deselnicu
Yulia Mishchenko
Daniel Brusse
Robert Forner
Dr. Scott Reid
Dr. Timothy Schmidt
Dr. Scott Kable
Funding:
$ National Science
Foundation
SEP
0.5 cm-1
SEP
14660
B(020)
14680
14700
14720
14740
14760