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 The goal of this experiment is to record a high resolution spectrum of the excited vibrational levels in formaldehyde to describe its potential energy surface. The conditions for recording Fluorescence Excitation (FE) and Dispersed Fluorescence (DF) spectra were studied and optimized. The sample was cooled in a free jet expansion to 6 K and excited with a Nd:YAG pumped dye laser 5 centimeters downstream to minimize collisional relaxation. Fluorescence was imaged into a monochromator with an ICCD detector resulting in a vibrtional spectrum of H 2 CO from 0 to 14,000 cm -1. The linewidth was 3 cm -1 and the signal-to-noise ratio was 5,300:1 at 4,000 cm -1 of vibrational energy. Assignment of the spectra is in progress. Future plans include applying this procedure to HDCO.

H 2 CO Normal Vibrational Modes 3N-6 = 6 different H 2 CO vibrations3N-6 = 6 different H 2 CO vibrations Measuring vibrational states characterizes the potential energy surface of the moleculeMeasuring vibrational states characterizes the potential energy surface of the molecule

Measuring Vibrational Energies Fluorescence Excitation Used to characterize S 1 energy levels s1s1s1s1 s0s0s0s0 ELELELEL Dispersed Fluorescence Used to characterize S 0 excited vibrational levels E V = E L - E F EFEFEFEF

Light: Lasers Advantages: monochromatic, directional, focusable, and intenseAdvantages: monochromatic, directional, focusable, and intense Allows excitation of a molecule to a single rovibronic quantum stateAllows excitation of a molecule to a single rovibronic quantum state

Molecules: The Molecular Beam pulsed nozzle Molecules have random speed and direction in nozzleMolecules have random speed and direction in nozzle Collisions during expansion result in uniform flowCollisions during expansion result in uniform flow Narrow velocity distribution results in a lower temperatureNarrow velocity distribution results in a lower temperature

Molecular Beam Cooler molecules produce better spectra because of a cleaner excitation (only excite to a single quantum state – no overlapping)

Detection: Monochromator & ICCD ICCD Detector Monochromator

Nozzle Height Three types of peaksThree types of peaks SignalSignal CollisionalCollisional NoiseNoise A height of 4 cm was chosen because it insured that there was little if no noise from collisionA height of 4 cm was chosen because it insured that there was little if no noise from collision

Slit Width Slit width is the width of the slit that allows light into the monochromatorSlit width is the width of the slit that allows light into the monochromator Controls the resolution of the signalControls the resolution of the signal A slit width of 150  m was chosen because it has the best signal:noise ratioA slit width of 150  m was chosen because it has the best signal:noise ratio

ICCD Settings Scan Number of Acquisitions Exposure Time (sec) Four scenarios of reading data:Four scenarios of reading data: From the graph the best choice is Scan 2 with 100 acquisitions at 1 second eachFrom the graph the best choice is Scan 2 with 100 acquisitions at 1 second each Binning is a way of collecting data and then transferring it over to the computerBinning is a way of collecting data and then transferring it over to the computer The two modes under consideration are:The two modes under consideration are: STR100STR100 STR200STR200 Since Readout is Scan 2, STR200 is a better choice for binningSince Readout is Scan 2, STR200 is a better choice for binning Readout Binning

Experimental Setup

4 1 H 2 CO

Assignments Vib LevelObservedExperimentalLiteratureTheoryObsv - Theor 1_1 2_3 4_ _3 4_3 5_ _3 4_ _1 4_ _2 4_ _1 4_ _ _ _

Future Complete assignments of the vibration states of H 2 COComplete assignments of the vibration states of H 2 CO Model the Potential Energy Surface of H 2 CO as a function of its geometryModel the Potential Energy Surface of H 2 CO as a function of its geometry Repeat the procedure for HDCORepeat the procedure for HDCO Understand how molecular weight and symmetry affect the vibration modes of a molecule by comparing H 2 CO, HDCO, and D 2 COUnderstand how molecular weight and symmetry affect the vibration modes of a molecule by comparing H 2 CO, HDCO, and D 2 CO Be able to predict the Potential Energy Surfaces of other moleculesBe able to predict the Potential Energy Surfaces of other molecules

Acknowledgements John Davisson and Mike PoublonJohn Davisson and Mike Poublon Hope College Chemistry DepartmentHope College Chemistry Department Research CorporationResearch Corporation Dreyfus FoundationDreyfus Foundation NSF-REUNSF-REU