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Beam Action Spectroscopy via Inelastic Scattering
BASIS Technique
Bobby H. Layne and Liam M. DuffyDepartment of Chemistry & Biochemistry, the University of North
Carolina at Greensboro, Greensboro, North Carolina 27402
Hans A Bechtel, Adam H. Steeves and Robert W. FieldDepartment of Chemistry, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
J. Phys. Chem. A (online)
Current Research
Using mm-Waves to probe:
• Photodissociation of atmospheric molecules:– characterizing quantum state distribution of products
– hyper-rovibronic detail
• Crossed molecular beams: reactive and inelastic scattering dynamics
Current Photodissociation Study
Chlorine Dioxide
h
Chlorine MonoxideOxygen
ClCl
OOO
O
Parent Molecule Products
OClO is an reservoir molecule for Cl radicals in the atmosphere
OClO Mode Specific UV Spectrum
0.0E+00
2.0E-18
4.0E-18
6.0E-18
8.0E-18
1.0E-17
1.2E-17
1.4E-17
1.6E-17
280 300 320 340 360 380 400 420 440
Wavelength (nm)
Ab
sorp
tion
Cro
ss S
ecti
on (
cm2 )
204 K
(14,
0,0)
(16,
0,0)
(18,
0,0)
(15,
0,0)
(17,
0,0)
(6,0
,0)
(5,0
,2)
(6,1
,0)
(1, 2, 3 )sym, bend, asym
(5,1
,2)
This Study
A. Wahner, G. S. Tyndall, and A. R. Ravishankara, J. Phys. Chem. 91, 2734 (1987).
mm-Wave: 1. Source Module2. Amplifier3. Multiplier4. Horn
Pulsed Slit Nozzle
Teflon Window & Lens
Top view of vacuum chamber with diffusion pump below
InSbHot Electron
Bolometer
ULN6pre
amp
Tunable UV from doubled OPO
4 3 2 1
CornerReflector MultipassCell
Photodissociation Setup
Current Available Range:50-330 GHz
Mm-WaveSource Module
“Armadillo”MicrowaveSynthesizer
Mm-WaveAmplifier &
Tripler
Frequency Resolution• 10 Hz at 100 GHz• or 1 part in 1010
Power > 1 mW
Parent Molecule
Photodissociationof Parent Molecule
Laser is fixed while mm-waves are stepped
UV Laser fixed toOClO (X 2B1 A 2A2 (15, 0, 0))
O35ClO hyperfine lines
Cl
OO
Products Probed in Hyper-rovibronic Detail
Cl
O
0
1
2
3
4
5
6
7
8
-500 -400 -300 -200 -100 0 100 200 300 400 500 600 700
Time (msec)
% A
bso
rpti
on
Hole
Millimeter-wave absorption time trace centered on a single hyperfine line of
O35ClO (268797.6550 MHz: N=65, J=6½5½, K-1=32, K+1=43, F=87)
Problem !
The “hole” shows a 50% depletion of the parent.
8.4% is expected from the product of laser fluence and UV cross-section.
“Hole burning” spectrum of O35ClO
30000 31000 32000 33000 34000 35000 36000
Frequency (cm-1)
Inte
nsi
ty (
arb
.)
. x 2.5
30000 31000 32000 33000 34000 35000 36000
Frequency (cm-1)
Inte
nsi
ty (
arb
.)
. x 2.5
(14,
0,0)
(15,
0,0)
(16,
0,0)
(17,
0,0)
?
BASIS spectrum of O35ClO and O37ClO
30650 30750 30850 30950 31050 31150 31250
Frequency (cm-1)
Inte
nsity
(arb
.)
y
35ClO
37ClO
O37ClO
O35ClOOCS
BASIS Signals
Products
Parent SignalBASIS Signal
SO2 Parent Hans Bechtel & Adam Steeves
(Field Group at MIT)
Low rotational transition313 - 202
High rotational transition817 - 808
artistic simulation
O C S
O C S
O C S
O C S
O C S
O C S O C S
O C S
O C S
O C S
O C S
O C S
O C S
Ar
Ar
Ar
Ar
Ar
Ar
ArAr
Ar Ar
Ar
Ar
Ar
ArAr
Ar
Ar
Ar
Ar
OCl
O
ArAr
UV BASIS Signals of OClO OCS reporter & fixed UV
J = 6
J = 25
Trot=22K
Raw time responses
Amplitudes from yellow cursors
Trot= 22 K
Trot= 25 K
Inte
nsit
yIn
tens
ity
diff
.
TransformedData (see settings)
Depletion
Buildup
Signals Inverted
J = 6
J = 25
Dynamic range 1:105
Observable rotational temperature shifts as small as 200 mK.
OCS acts as a “virtual bolometer”
Infrared BASIS Hans Bechtel & Adam Steeves
(Field Group at MIT)
H-C C-H + IR H-C C-H(vib. cold) (vib. hot)
Reporting Molecule
H-C C-H + OCS H-C C-H + OCS (vib. cold)(vib. hot) (rot. hot)(rot. cold)
collision
near nozzle
IR-BASIS Spectrum of Acetylenemonitored via OCS
BASIS Advantages
• Gain: – energy deposited in beam is large– e.g. 6x over traditional hole burning (f )– analogous to optothermal technique (but simpler)– may extend the sensitivity of direct-IR absorption
• General Method:– should work for any rotationally resolved
molecular beam spectroscopic technique
Best BASIS Conditions
• Cold rotational distribution
• High density region:– reporting molecule needs to stay in the beam– near nozzle for IR BASIS– down stream possible for slit jet photodissociation
• Reporting molecule chosen:– largest rotational line intensity – does not photodissociate– large RT collision cross section?
Possible BASIS Experiments• UV, Vis, IR – BASIS• Dark states• Surface BASIS:
– scattering off of optically excited SAMs
Side Implications• Slit-jet densities are so large that fragments are
entrained even 10 cm downstream• Pump-probe time delay important, particularly in CW
experiments– Lifetime broadening may be collisional broadening.
(e.g. OClO near Frank-Condon max)
Acknowledgements
UNCG Undergraduate:Bobby H. Layne
Hans A Bechtel, Adam H. Steeves and Robert W. Field
H.A.B. acknowledges the Donors of the AmericanChemical Society Petroleum Research Fund for support, andA.H.S. acknowledges the Army Research Office for a NationalDefense Science and Engineering Graduate Fellowship. The
work at MIT was supported by the Office of Basic EnergySciences of the U.S. Department of Energy
Helpful conversations & BASIS acronym:Prof. Robert M. Whitnell