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Brookhaven Science AssociatesU.S. Department of Energy
Hot band transitions in CH2Hot band transitions in CH2
Kaori Kobayashi*, Trevor Sears, Greg Hall
Department of Chemistry
Brookhaven National Laboratory
Thanks to:Prof. Hideto Kanamori, Tokyo Institute of Technology
Funding:
DOE Division of Chemical Sciences, Office of Basic Energy Sciences
*Current address, Dept. of Physics, Toyama University
Brookhaven Science AssociatesU.S. Department of Energy
OutlineOutline
• Near-IR spectrum of CH2
– Renner-Teller states a, b– Spectra involving levels near b-state origin
• Transitions from a(010)– Rotational structure within this level– Perturbing triplet state (X) levels
• Observation of c-state of CH2
Brookhaven Science AssociatesU.S. Department of Energy
Electronic states of CH2Electronic states of CH2
H
H1B1
H
H1A1
H
H3B1
• C: 4 valence electrons• 2 used in bonding to 2 H
– 2 left to put into 2 orbitals
• Leads to 3 low-lying electronic states– Lowest is triplet: electrons with spins
aligned– Two more with zero net electron spin
• Different chemical and physical properties– Potential energy surfaces govern
properties
(a)
(X)
(b)
Brookhaven Science AssociatesU.S. Department of Energy
Near-IR spectrum of CH2Near-IR spectrum of CH2
0.0
-0.5
0.5
1.0
1.5
2.0
2.5
3.0x104
/degrees
CH2 bending potentialcurves
X (Triplet)
a (Singlet)
b(Singlet)
• Two singlet states-correlate with a 1Δ state at linearity
• Transitions between them result in band system in near-IR and visible
• Perturbations due to mutual interactions (RTE) and background of triplet (X) state levels
Cuts through the lowest three electronic PES’s along bending coordinate
Brookhaven Science AssociatesU.S. Department of Energy
Time-resolved FM absorptionTime-resolved FM absorption
Sample Cell
Excimer
Laser
CW ringAr Laser
PC
receiver
Scope
+EOM
Digital
time
Photo
RF
CH2CO + 308nm → CH2 + CO
Ti:sapphire laser replaced by tunable diode operating near 1.3 microns
Shot noise sensitivity
Sub-microsec. time resolution, Doppler-limited frequency resolution
Brookhaven Science AssociatesU.S. Department of Energy
Transitions near 7400 cm-1Transitions near 7400 cm-1
• Below the b-a band origin, only hot bands or overtone transitions– Access region close to
linearity from a(010)
• Ortho and para CH2 3:1 intensity ratio easy to see here
• Spectra relatively uncomplicated, compared to shorter wavelengths 7454 7455 7456 7457 7458 7459
wavenumbers/cm-1
-2
-1
0
1
2
b(000)2-a(010)1~ ~422-414
523-515
624-616
321-313
220-212
725-717
422-414
Brookhaven Science AssociatesU.S. Department of Energy
Observed and calculated levelsObserved and calculated levels
Level K Experiment Gu et al. Green et al. Duxbury et al.
b~ (0,1,0) 0 9537 9537 9566 9539
b~ (0,2,0) 0 10827 10831 10848 10822
a~ (0,7,0) 1 8725 8717 - - a~ (0,8,0) 1 9444 9450 - - a~ (1,7,0) 1 11253 11242 11292 11268
b~ (0,2,0) 1 11355 11364 11378 11352
b(0,0,0) 2 8871 8870 8901 8820 b(0,0,0) 4 9711 9754 9716 9621
b~ (0,2,0) 4 12870 12878
12995 12873 12856
(cm-1)
Also (0,10,0)2
Brookhaven Science AssociatesU.S. Department of Energy
ã(0,1,0) state rotational levelsã(0,1,0) state rotational levels
• Accumulated combination differences for ã(0,1,0) obtained from multiple bands as follows:
• b(0,0,0) K = 2
• b(0,2,0) K = 0, 1, 2, 4
• a(0,7,0) K = 1
• a(0,10,0) K = 1, 3
Brookhaven Science AssociatesU.S. Department of Energy
Fit ã(010) combination differencesFit ã(010) combination differences
Brookhaven Science AssociatesU.S. Department of Energy
K=1 levels perturbedK=1 levels perturbed
-0.7-0.6-0.5-0.4-0.3-0.2-0.1
00.10.20.30.4
0 10 20 30 40 50
J (J +1)
Shift
(wav
enum
ber)
J 1,J -1J 1J
Brookhaven Science AssociatesU.S. Department of Energy
Perturbing levels? (vibronic origins)Perturbing levels? (vibronic origins)
4100
4200
4300
4400
4500
4600
4700
4800
4900
5000
5100
Ka = 1 Ka = 2Ka = 0 Ka = 3 Ka = 4
1CH2 (0,1,0)
1CH2 (0,1,0)
1CH2 (0,1,0)
1CH2 (0,1,0)
1CH2 (0,1,0)
3CH2 (0,4,0)
3CH2 (0,3,0)
3CH2 (0,3,0)
3CH2 (1,1,0)
3CH2 (1,1,0)
3CH2 (0,1,1)
3CH2 (0,1,1)
3CH2 (0,1,1)
3CH2 (0,1,1)
3CH2 (0,0,1)
3CH2 (1,1,0)
Brookhaven Science AssociatesU.S. Department of Energy
Rotational levelsRotational levels
4400
4500
4600
4700
4800
4900
5000
Ener
gy (
wave
numb
er)
K=1 a(010) K=1 X(040) K=3 X(030) K=2 X(011)
23
4
6
3
4
5
6
7
234
5
Observed
Brookhaven Science AssociatesU.S. Department of Energy
CH2 c-stateCH2 c-state
• Dominant electron configurations, wavefunctions contain both– “Forbidden” c-a transition
• OODR experiment
H
H1A1
H
H1A1
(c)
(a)
0
10000
20000
30000
c (0,0,0)~
b (0,0,0)~
b (0,14,0)0,2~
c (0,11,0)1~
a (0,0,0)1~
near-ir
visible
Brookhaven Science AssociatesU.S. Department of Energy
OODR SpectraOODR Spectra
Pump b-a transition with ns dye laser approx 0.4 microsec after photolysis pulse. Record absorption of FM’d Ti:sapphire laser.
Brookhaven Science AssociatesU.S. Department of Energy
c-state assignmentsc-state assignments
• Rotational levels in (0,11,0) of c1A1
– All K=1 levels accessible from an ambient temperature sample of a1A1 CH2
– Some strong localized perturbations
• OODR technique applicable to search for other higher CH2 electronic states
– Particularly d 1A2 that is predicted to have a very small bond angle
Brookhaven Science AssociatesU.S. Department of Energy
FinallyFinally
Greg Hall and Anatoly Komissarov
…and other members of the GPMD group at BNL
Additional people who contributed to the work….
Yangsoo Kim