Spectroscopy of Highly Excited Vibrational States of Formaldehyde by Dispersed Fluorescence

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Spectroscopy of Highly Spectroscopy of Highly Excited Vibrational States Excited Vibrational States of Formaldehyde by Dispersed of Formaldehyde by Dispersed Fluorescence Fluorescence Jennifer D. Herdman, Brian D. Lajiness, Jennifer D. Herdman, Brian D. Lajiness, James P. Lajiness, and William F. Polik James P. Lajiness, and William F. Polik Hope College, Holland, MI Hope College, Holland, MI Summer 2004 Summer 2004

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Spectroscopy of Highly Excited Vibrational States of Formaldehyde by Dispersed Fluorescence. Jennifer D. Herdman, Brian D. Lajiness, James P. Lajiness, and William F. Polik Hope College, Holland, MI Summer 2004. Abstract. - PowerPoint PPT Presentation

Transcript of Spectroscopy of Highly Excited Vibrational States of Formaldehyde by Dispersed Fluorescence

Page 1: Spectroscopy of Highly Excited Vibrational States of Formaldehyde by Dispersed Fluorescence

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

Page 2: Spectroscopy of Highly Excited Vibrational States of Formaldehyde by Dispersed Fluorescence

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.

Page 3: Spectroscopy of Highly Excited Vibrational States of Formaldehyde by Dispersed Fluorescence

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

+

+ +

-

Page 4: Spectroscopy of Highly Excited Vibrational States of Formaldehyde by Dispersed Fluorescence

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

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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

Page 6: Spectroscopy of Highly Excited Vibrational States of Formaldehyde by Dispersed Fluorescence

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

Page 7: Spectroscopy of Highly Excited Vibrational States of Formaldehyde by Dispersed Fluorescence

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

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Detection: Monochromator Detection: Monochromator && ICCD ICCD

ICCD DetectorICCD Detector

MonochromatorMonochromator

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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

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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

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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

Page 12: Spectroscopy of Highly Excited Vibrational States of Formaldehyde by Dispersed Fluorescence

Experimental SetupExperimental Setup

Sign

al

Nd: YAG Laser

Tunable Dye Laser

ICCD

Computer

Doubling Crystal

Filter

H2CO + Ne

Monochromator

Frequency

Page 13: Spectroscopy of Highly Excited Vibrational States of Formaldehyde by Dispersed Fluorescence

0 2000 4000 6000 8000 10000 12000

Energy

Inte

nsit

y4411 H H22COCO

Page 14: Spectroscopy of Highly Excited Vibrational States of Formaldehyde by Dispersed Fluorescence

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

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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

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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