Molecular Triplet States: Excitation, Detection, and Dynamics

Wilton L. VirgoKyle L. BittingerRobert W. Field

Collisional Excitation Transfer in the Xe*-N2 System: Proxies for Hg*-acetylene,ethylene

Why Triplet States ?

Reactive (E* 100 kcal/mol) Long-lived ( > 100 s) Difficult to detect (No UV fluorescence) Properties differ from ground state Easily populated unintentionally Unknown: Structure

Excitation mechanisms

Decay mechanisms

Photosensitized Excitation Transfer Our Goal: use atomic photosensitization,

exciting atoms via 2-photon optical pumping Hg* + C2H2 Hg + C2H2*

Xe* + N2 Xe + N2*

A Pulsed Beam Source of Metastable Molecules

Excite an electron on a

closed-shell (1S0) atom

into p orbital

L=1 , S=1,0

J=2,1,0 (L+S,...,0)

Terms: 1P1, 3P0, 3P1, 3P2

Order of triplet sublevels: sign of spin-orbit constant

Hg: 0,1,2 normal

Xe: 2,1,0 inverted

1P1 decays to ground state

3P2 or 0 metastable

3P1 mixes with 1P1 decays

3P0 or 2 metastable

Metastable States of Closed-Shell Atoms

Excite to short-lived 3D2

state via two-photon transition at 252nm

Decays in 28 nsec to the lowest two excited states:

63D2 two-photon

pump state

2 photon transition from ground state

New Optical Pumping Scheme for Populating Xe (3P2)

33% to 3P1 (895nm, 10 ns) decays to ground state

67% to 3P2 (823nm, 150 s)metastable state

Detect N2* B3g A3u emission.

(5,3) and (5,2) bands dominant

Krumpelmann CPL 140, 142 (1987)

Previous Studies of Xe* + N2 by OttingerExcitation Transfer Detected via Dispersed

Fluorescence

Excite Xe by electron

impact or electrical discharge

Excitation transfer via

Xe beam / N2 gas target

or crossed beam

Two Methods of Detection

LIF Sensitive to short-lived states < 10s Determine the number of metastables

produced

SEELEM (Surface Electron Ejection by Laser Excited Metastables) Sensitive to long-lived states > 600s Time-of-flight spectra

SEELEM: Electronic De-Excitation at Metal Surface

Criterion for e- emission: Eel > metal (work function)5.1 eV (Au)

Surface Electron Ejection by Laser Excited Metastables

e-

AuSurface

Co-expand a mixture of Xe and N2

Excitation Transfer in the Molecular Beam

0.1050.008

0.015

0.082

0.0530.151

0.002

Possible Xe (3P2)N2 Metastable ResonancesAnd Franck-Condon Factors

0.096

Xe and N2 LIF:Signals on two different timescales

Xe (3D23P2) @ 823 nm ~30ns

N2 (B3g A3u) @748 nm ~5s

Excitation in Post-Expansion Region .75” in front of Nozzle

Time-of-Flight SEELEM: ‘Slow Collisions’

TOF-SEELEMExcitation in Expansion Region

50 PSIbacking pressure

TOF-SEELEM 120 PSI Backing Pressure

How Well Are We Doing?

0.01 bar, 1 mm3 1014 Xe atoms 2-photon 1% saturated 1012 Xe* Observe 1x106 Xe*, 1x106 N2*

SEELEM Counts: 2500 each Xe* & N2* Xe*+Xe* Xe + Xe+ + e-

Penning Ionization? Associative ionization to Xe2

+ + e- ?

Future Experiments

LIF probe of N2* states 3 Photon excitation of Xe*, Kr*, etc. Ablation jet for Hg*, Cd*, Zn* Hg* on acetylene and ethylene

Acknowledgements

Prof. Robert W. Field Kyle Bittinger Sam Lipoff Jessica Lam AFOSR

The Ultimate Goal: Hg/Acetylene & EthyleneLaser Ablation to the Rescue !

Hg Reservoir

and Acetylene too !

Ablation Pulse

Orbital Mechanism of Excitation Transfer

Xe 5p-1 6sN2 g g*

Hg 6s 6pHCCHu g*

Detection of Xe and N2 Metastables via Fluorescence

The total charge collected…

is the number of excited species

…times the efficiency of the optics

…times the quantum efficiency of the detector at each fluorescence wavelength

…times the gain of the detector and the electron charge

Laser Induced Fluorescence of Xe + N2: Estimating Excitation Transfer Efficiency

Excitation transfer efficiency: calculate the relative number of

Xe, N2 molecules observed during

simultaneous measurement

Many factors are the same in both measurements:

Geometry of optics Laser power Gain of detector Resistance of detector circuit

Number of molecules observed is a function of:

Vave t charge collected

Qe at 823nm, 748nm, 677nm

dtV

RR

tVave 1

en

216

1

F

eG

,eQ

Rearrange equations for ne and

remove constant factors

Calculation based on relative band intensities observed in similar experiments

Laser Induced Fluorescence of Xe + N2: Excitation Transfer Efficiency Calculation

)108(

16

16

1

12

,

1

,

2

,2

e

ave

eavee

eeave

Q

tV

QeGR

FtVn

eGQF

nR

tV

Xe 3D2 3P2 823 nm Qe = 0.21%

N2 B A (4,2)

A (4,1)

748 nm677 nm

1 % 3 %

1% NO2 in He625 Torr backing pressure

90 shot averaging

Speed of beam:

1800 m/s

Doppler broadening limit

using 3mm skimmer:

0.007 cm-1

Measured Doppler

broadening:

0.010 cm-1

NO2 spectra recorded with frequency-doubled CW ring laser

Franck-Condon factors for low-lying excited states of N2

v’ FC factor

E, cm-

1

5 0.1054 475

4 0.1512 -1114

3 0.1907 -2732

2 0.1954 -4380

1 0.1477 -6056

0 0.06105 -7761

v’ FC factor

E, cm-1

6 0.0960 831

5 0.0818 -528

4 0.0618 -1910

3 0.0397 -3318

2 0.0204 -4749

1 0.00746 -6206

0 0.00146 -7687

B3g W3u

v’ FC factor

E, cm-

1

1 0.00802 277

0 0.00158 -1216

B’3u-

v’ FC factor

E, cm-

1

14 0.01487 164

13 0.0532 -891

12 0.0638 -1979

11 0.0744 -3096

10 0.0840 -4245

9 0.0910 -5423

8 0.0939 -6631

7 0.0911 -7867

6 0.0819 -9133

5 0.0670 -10427

4 0.0485 -11748

A3u+-X1+

g (v’’=0)

Gilmore, Laher, and Espy. J Phys Chem Ref Data 21, 1005 (1992)Lofthus and Krupenie. J Phys Chem Ref Data. 6, 113 (1977)

Xe 3P2

energy

Previous Studies of Xe + N2 Excitation Transfer

3P2 state of xenon lies 475 cm-1

below v=5 and 1114 cm-1 above

v=4 of N2 B3g state.

Energy transfer into v=5 occurs w/absolute cross-section of 12.5A2 at avg. collision energy of 452cm-1

a) Ottinger Chem Phys 192, 49 (1995)Krumpelmann CPL 140, 142 (1987)

N2 B3g

Levels

Laser tuned to Xe 63D2 61S0

two-photon transition

Detect Xe* by fluorescence to metastable state at 823nm

(used 610nm long-pass filter)

We have done this before in a cell, but this was the first time for us in the molecular beam

Fluorescence lifetime is comparable to detector response time (28ns)

Two-photon transition probability ~1/10 that of comparable transition in Hg

Laser Induced Fluorescence of Xe* + N2: Preparing Metastable Xe