Electronic spectroscopy of DCFElectronic spectroscopy of DCF

Haiyan Fan, Mihaela Deselnicu, Chong Tao, Calvin Mukarakate, Ionela Ionescu and Scott Reid*

Department of Chemistry, Marquette University, P. O. Box 1881 Milwaukee, WI 53201-1881

60th International Symposium on Molecular Spectroscopy

Ene

rgy

HCF: A case study for the RT effect

HCF represents a case of the Renner-Teller (RT) effect whereboth states have large barriers to linearity(Other Examples: CCl2, SiH2)

Why is this interesting?The RT interaction is largest near the barrier region, where the wavefunctions overlap most strongly.Thus, one can view the onset of theRT interaction.

In HCF, the two singlet states correlate with 1 in the linearconfiguration.

X1A

Ã1Aã3A

HCF: A case study for the RT effect

0

20

40

60

80

100

120

140

0 2 4 6 8

2'

A r

otat

iona

l con

stan

t in

cm-1

(1,n,0)

(0,n,0)

(0,n,1)

0

1

2

3

4

5

6

7

8

9

0 1 2 3 4 5 6 7 8

2'L

ifetim

e in

ms

K=0

K=1

K=2

In HCF, the onset of the barrier to linearity is observed in: an increase of A rotational constant with bending excitation fluorescence lifetime lengthening for K ≥ 1, due to onset of strong RT effects as the barrier is approached

Building upon our previous studies of HCF, we wish to probe the spectroscopy and dynamics of the Renner-Teller effect in DCF using:Building upon our previous studies of HCF, we wish to probe the spectroscopy and dynamics of the Renner-Teller effect in DCF using:

Fluorescence Excitation SpectroscopyFluorescence Excitation Spectroscopy

Fluorescence lifetime measurementsFluorescence lifetime measurements

Polarization Quantum Beat spectroscopyPolarization Quantum Beat spectroscopy

Determine excited state barriers to linearity and dissociationDetermine excited state barriers to linearity and dissociation Compare our results with high level electronic structure calculationsCompare our results with high level electronic structure calculations

Objectives of the present work

Experimental Details

DCF is generated from CD3F using a pulsed discharge source and probed

using laser induced fluorescence

Lifetime measurements use on-axis geometry to minimize effects from fly-out

GHV

laser

Fluorescence excitation spectrum of DCF: (0,0,0)

K

01

2

01

2

Trot ~ 15 K17280 17300 17320 17340 17360 17380 17400 17420

Simulation

Inte

nsity

wavenumber (cm-1)

Experiment

3

Fluorescence excitation spectra of (0,2,0) states

2 = 5

3

2

1

0

-20 0 20 40 60 80 100 120

wavenumber relative to band origin (cm-1)

4

Variation in A rotational constant with quanta of bend for à state

The A constant increases dramatically with bending excitation, signifying the approach to linearity

a

A Ia-1

small A

large A

0 1 2 3 4 5 6

15

20

25

30

35

40

2

A c

onst

ant

in c

m-1

Variation in vibrational intervals for pure bending states (0,2,0), K = 0

“Dixon dip” occurs at barrier to linearity

0 2 4 6 8

720

730

740

750

760

770

780

790G

(2+

1)-

G(

2)

Dixon plot of vibrational intervals for pure bending states (0,2,0), K = 0

Quadratic fit yields a barrier height of 6660 430 cm-1

Recent theoretical estimate: 6770 cm-1

Schmidt, et al. Chem. Phys. Lett. 292, 80 (1998)

22000 23000 24000 25000 26000

724

726

728

730

732

734

736

738

740

742

G(

+1

)-G

( )

0.5*[G()+G(+1)]

E = 6660 cm-1

à state vibrational parameters for HCF and DCF

Parameter

HCF

(in cm-1)

DCF

(in cm-1)

2784.9(24) 2113.6(16)

´ 1029.9(8) 792.1(2)

´ 1270.8(17) 1254.0(5)

x12´ - 36.6(6) - 15.1(4)

x22´ - 7.5(2) - 4.54(3)

x23´ - 10.0(10) - 16.3(8)

Probing the barrier to dissociation

The highest level observed to date using LIF is the Ka = 0 level of (1,8,0), corresponding to a vibrational energy of 8048 cm-1 above the vibrationless level of the à state

In HCF, the highest level we observed using LIF is the Ka = 1 level of (1,6,0), corresponding to a vibrational energy of 8555 cm-1 above the vibrationless level of the à state

We thus set a lower limit on the dissociation energy in the à state of 8555 cm-1, which is consistent with a recent theoretical estimate of 8955 cm-1

Schmidt, et al. Chem. Phys. Lett. 292, 80 (1998)

Summary and Conclusions

The Renner-Teller effect in DCF has been investigated through fluorescence excitation spectroscopy

The onset of the barrier to linearity is observed in:• a minimum in the bending vibrational intervals• an increase of A rotational constant with bending excitation

The derived barrier height is in excellent agreement with theory

Acknowledgements

People:

Haiyan Fan (Ph.D. 2004)

Ionela Ionescu (M.S. 2004)

Eduard Ionescu (M.S. 2004)

Chris Annesley (undergrad)

Joseph Cummins (undergrad)

Matthew Bowers (undergrad)

Dr. Ju Xin (Bloomsburg U.)

Funding:

NSF

ACS/PRF