Brookhaven Science Associates U.S. Department of Energy Hot band transitions in CH 2 Kaori Kobayashi...

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


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


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)


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


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


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


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


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


Fit ã(010) combination differencesFit ã(010) combination differences


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


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)


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


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


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.


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


FinallyFinally

Greg Hall and Anatoly Komissarov

…and other members of the GPMD group at BNL

Additional people who contributed to the work….

Yangsoo Kim

Brookhaven Science Associates U.S. Department of Energy Hot band transitions in CH 2 Kaori Kobayashi...

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