Spectroscopy of Highly Excited Vibrational States of Formaldehyde by Dispersed Fluorescence

1. 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

2. 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 H2CO 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.

3. H2CO Normal Vibrational Modes • 3N-6 = 6 different H2CO vibrations • Measuring vibrational states characterizes the potential energy surface of the molecule

4. Measuring Vibrational Energies Fluorescence Excitation Dispersed Fluorescence s1 EF EL s0 EV = EL- EF Used to characterize S1 energy levels Used to characterize S0 excited vibrational levels

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

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

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

8. Detection: Monochromator & ICCD Monochromator ICCD Detector

9. Nozzle Height • Three types of peaks • Signal • Collisional • Noise • A height of 4 cm was chosen because it insured that there was little if no noise from collision

10. Slit Width • Slit width is the width of the slit that allows light into the monochromator • Controls the resolution of the signal • A slit width of 150 mm was chosen because it has the best signal:noise ratio

11. ICCD Settings Readout Binning • Four scenarios of reading data: • Binning is a way of collecting data and then transferring it over to the computer • The two modes under consideration are: • STR100 • STR200 • Since Readout is Scan 2, STR200 is a better choice for binning • From the graph the best choice is Scan 2 with 100 acquisitions at 1 second each

12. Experimental Setup

13. 41 H2CO

14. Assignments

15. Future • Complete assignments of the vibration states of H2CO • Model the Potential Energy Surface of H2CO as a function of its geometry • Repeat the procedure for HDCO • Understand how molecular weight and symmetry affect the vibration modes of a molecule by comparing H2CO, HDCO, and D2CO • Be able to predict the Potential Energy Surfaces of other molecules

16. Acknowledgements • John Davisson and Mike Poublon • Hope College Chemistry Department • Research Corporation • Dreyfus Foundation • NSF-REU