Spectroscopy of Highly Excited Spectroscopy of Highly Excited Vibrational States of Vibrational States of
Formaldehyde by Dispersed Formaldehyde by Dispersed FluorescenceFluorescence
Jennifer D. Herdman, Brian D. Lajiness, James P. Jennifer D. Herdman, Brian D. Lajiness, James P. Lajiness, and William F. PolikLajiness, and William F. Polik
Hope College, Holland, MIHope College, Holland, MI
Summer 2004Summer 2004
AbstractAbstractThe goal of this experiment is to record a high The goal of this experiment is to record a high resolution spectrum of the excited vibrational levels resolution spectrum of the excited vibrational levels in formaldehyde to describe its potential energy in formaldehyde to describe its potential energy surface. The conditions for recording Fluorescence surface. The conditions for recording Fluorescence Excitation (FE) and Dispersed Fluorescence (DF) Excitation (FE) and Dispersed Fluorescence (DF) spectra were studied and optimized. The sample spectra were studied and optimized. The sample was cooled in a free jet expansion to 6 K and excited was cooled in a free jet expansion to 6 K and excited with a Nd:YAG pumped dye laser 5 centimeters with a Nd:YAG pumped dye laser 5 centimeters downstream to minimize collisional relaxation. downstream to minimize collisional relaxation. Fluorescence was imaged into a monochromator with Fluorescence was imaged into a monochromator with an ICCD detector resulting in a vibrtional spectrum of an ICCD detector resulting in a vibrtional spectrum of HH22CO from 0 to 14,000 cmCO from 0 to 14,000 cm-1-1. The linewidth was 3 cm. The linewidth was 3 cm--
11 and the signal-to-noise ratio was 5,300:1 at 4,000 and the signal-to-noise ratio was 5,300:1 at 4,000 cmcm-1-1 of vibrational energy. Assignment of the of vibrational energy. Assignment of the spectra is in progress. Future plans include applying spectra is in progress. Future plans include applying this procedure to HDCO. this procedure to HDCO.
HH22CO Normal Vibrational CO Normal Vibrational ModesModes
•33NN-6 = 6 -6 = 6 different Hdifferent H22CO CO vibrationsvibrations
•Measuring Measuring vibrational states vibrational states characterizes the characterizes the potential energy potential energy surface of the surface of the moleculemolecule
Symmetric C-H Stretch
C
O
H H
C=O Stretch
C
O
H H
C-H2 Bend
C
O
H H
C
O
H H
C
O
H H
Antisymmetric C-H Stretch
C
O
H H
C-H2 Rock Out-of-plane Bend
+
+ +
-
Measuring Vibrational Measuring Vibrational EnergiesEnergies
Fluorescence ExcitationFluorescence Excitation
Used to Used to characterize characterize
SS11 energy levels energy levels
ss
11
ss
00
EELL
Dispersed FluorescenceDispersed Fluorescence
Used to characterize Used to characterize SS00 excited vibrational excited vibrational
levelslevels
EEVV = E = ELL- E- EFF
EE
FF
Light: LasersLight: Lasers
• Advantages: Advantages:
monochromatic, monochromatic,
directional, directional,
focusable, and focusable, and
intense intense
• Allows excitation Allows excitation
of a molecule to a of a molecule to a
single rovibronic single rovibronic
quantum statequantum state
Molecules: The Molecular Molecules: The Molecular BeamBeam
pulsed nozzlepulsed nozzle
•Molecules have random Molecules have random speed and direction in speed and direction in nozzlenozzle
•Collisions during Collisions during expansion result in uniform expansion result in uniform flowflow
•Narrow velocity Narrow velocity distribution results in a distribution results in a lower temperaturelower temperature
Molecular BeamMolecular Beam
Backing Pressure vs Temperature
y = 6.226x-0.5307
R2 = 0.9839
0
10
20
30
0.0 0.5 1.0 1.5 2.0
Backing Pressure (atm)
Tem
pera
ture
(K)
Cooler molecules produce Cooler molecules produce better spectra because of a better spectra because of a
cleaner excitation (only excite cleaner excitation (only excite to a single quantum state – no to a single quantum state – no
overlapping)overlapping)
22.37 K BP .1 atm
28320 28322 28324 28326 28328Frequency
Inte
nsi
ty
6.70 K BP 1.0 atm
28320 28322 28324 28326 28328
Frequency
Inte
nsit
y
Room Temperature
28320 28322 28324 28326 28328
Frequency
Inte
nsity
Detection: Monochromator Detection: Monochromator && ICCD ICCD
ICCD DetectorICCD Detector
MonochromatorMonochromator
Nozzle HeightNozzle Height
Nozzle Height
0
200
400
600
800
0 1 2 3 4 5 6 7 8
Nozzle Height (cm)
Noi
se R
atio
s
Signal:Noise
Collision:Noise
Nozzle Height 2 cm
0 100 200 300 400 500
Pixels
Inte
nsit
y
•Three types of peaks Three types of peaks •SignalSignal•CollisionalCollisional•NoiseNoise
•A height of 4 cm was chosen A height of 4 cm was chosen because it insured that there because it insured that there was little if no noise from was little if no noise from collisioncollision
Nozzle Height 4 cm
0 100 200 300 400 500
Pixels
Inte
nsit
y
Slit WidthSlit Width25 m
0 100 200 300 400 500
Pixels
Inte
nsit
y
100 m
0 100 200 300 400 500
PixelsIn
tens
ity
•Slit width is the width of the slit Slit width is the width of the slit that allows light into the that allows light into the monochromatormonochromator
•Controls the resolution of the Controls the resolution of the signalsignal
•A slit width of 150 A slit width of 150 m was m was chosen because it has the best chosen because it has the best signal:noise ratiosignal:noise ratio
400 m
0 100 200 300 400 500
Pixels
Inte
nsit
y
Height:Full Width Half Max
0
50000
100000
150000
0 100 200 300 400 500
Slit Width
Hei
ght:
FWH
M
ICCD SettingsICCD Settings
ScaScann
Number of Number of AcquisitioAcquisitio
nsns
Exposure Exposure Time Time (sec)(sec)
11 10001000 .005.005
22 100100 11
33 1010 1010
44 11 100100
Signal:Noise
0
1000
2000
3000
1 2 3 4
Scan
Sign
al:N
ois
e
•Four scenarios of reading data:Four scenarios of reading data:
•From the graph the best choice is From the graph the best choice is Scan 2 with 100 acquisitions at 1 Scan 2 with 100 acquisitions at 1 second eachsecond each
•Binning is a way of collecting data Binning is a way of collecting data and then transferring it over to the and then transferring it over to the computercomputer
•The two modes under consideration The two modes under consideration are:are:
•STR100STR100•STR200STR200
•Since Readout is Scan 2, STR200 is Since Readout is Scan 2, STR200 is a better choice for binninga better choice for binning
Number of Data Points
0.00
50.00
100.00
150.00
200.00
250.00
300.00
1 2 3 4
Scan
Sign
al:N
oise
STR100
STR200
ReadoutReadout BinningBinning
Experimental SetupExperimental Setup
Sign
al
Nd: YAG Laser
Tunable Dye Laser
ICCD
Computer
Doubling Crystal
Filter
H2CO + Ne
Monochromator
Frequency
0 2000 4000 6000 8000 10000 12000
Energy
Inte
nsit
y4411 H H22COCO
AssignmentsAssignments
Vib Level ObservedExperiment
al Literature TheoryObsv - Theor
1_1 2_3 4_4 12461.1 12470.2 - 12460.4 0.72
2_3 4_3 5_1 11342.9 11343.1 - 11341.3 1.58
2_3 4_2 7461.8 7462.6 7462.0 7462.4 -0.60
2_1 4_4 6344.1 6346.2 6345.0 6345.3 -1.20
2_2 4_2 5768.1 5768.8 5769.0 5769.2 -1.10
2_1 4_2 4057.2 4058.3 - 4058.2 -0.99
2_2 3471.5 3471.6 3471.7 3472.4 -0.88
2_1 1746.0 1746.1 1476.0 1746.1 -0.12
4_0 -0.5 -0.3 0.0 0.0 -0.55
FutureFuture
•Complete assignments of the vibration states of Complete assignments of the vibration states of HH22COCO
•Model the Potential Energy Surface of HModel the Potential Energy Surface of H22CO as a CO as a function of its geometryfunction of its geometry
•Repeat the procedure for HDCORepeat the procedure for HDCO
•Understand how molecular weight and Understand how molecular weight and symmetry affect the vibration modes of a symmetry affect the vibration modes of a molecule by comparing Hmolecule by comparing H22CO, HDCO, and DCO, HDCO, and D22COCO
•Be able to predict the Potential Energy Surfaces Be able to predict the Potential Energy Surfaces of other moleculesof other molecules
AcknowledgementsAcknowledgements
•John Davisson and Mike PoublonJohn Davisson and Mike Poublon•Hope College Chemistry DepartmentHope College Chemistry Department
•Research CorporationResearch Corporation•Dreyfus FoundationDreyfus Foundation
•NSF-REU NSF-REU