Dispersed Fluorescence Spectroscopy of D2CO

Gregory D. Martin, Todd P. Chassee, Tyson O. Friday, and William F. Polik

Hope College, Holland, MI 49423, USA

http://www.chem.hope.edu/~polik/poster/d2co96.htm

Introduction

The Potential Energy Surface (PES) provides a fundamental description of a reaction mechanism. The PES can be characterized by measuring the vibrational level spectrum.

Reaction Coordinate Figure

Dispersed Fluorescence (DF) spectroscopy measures vibrational levels resulting in a better understanding of the PES. The current work on D2CO extends previous work on H2CO.

Dispersed Fluorescence (DF) Spectroscopy

At high energies, vibrational transitions are extremely weak and overlapped due to rotational congestion. These difficulties can be overcome by recording DF spectra in a molecular beam.

Dispersed Fluorescence Method Figure

Rotational congestion is removed by exciting the 000 rotational level of a S1 vibronic level. High sensitivity is achieved because dispersed fluorescence is a zero-background technique. The energies of vibrational levels are given by

Evib = Elaser - Efluorescence

Experimental Setup

Light, molecules, and detection are the components of DF spectroscopy.

Experimental Setup Figure

A Nd:YAG laser pumps a tunable dye laser. The laser light is converted to ultraviolet (UV) light, filtered, and directed into the molecular beam chamber. The D2CO molecules are seeded in helium, cooled in a molecular beam, and excited by the UV laser light. The fluorescence is imaged into a monochromator and dispersed onto an Intensified Charged Coupling Device (ICCD) detector. The DF spectrum is analyzed by a computer.

Results

The energy spectrum of D2CO fluorescing from the 000 rotational level in the S1 41 vibronic state is presented below. Vibrational states are observed well above 10000 cm-1 in energy.

41 DF Spectrum

Peaks below 3000 cm-1 have been assigned using the H2CO potential energy surface and symmetry-based selection rules.

Assigned 41 DF Spectrum

Assignment Experiment Literature Theory Experiment
- Theory
00 0.6 0.0 0.0 0.6
41 938.7 938.0 938.5 0.2
61 993.7 989.3 989.2 4.5
31   1105.7 1100.3  
21 1702.0 1701.6 1699.9 2.1
42 1867.5   1868.7 -1.2
4161 1929.8   1929.8 0.0
62 1976.3   1974.9 1.4
3141     2038.9  
11 2061.3 2055.8 2061.0 0.4
3161     2072.1  
51 2163.3 2159.7 2160.7 2.7
32     2201.6  
2141 2634.7   2632.5 2.2
2161 2685.7   2684.1 1.5
43 2790.8   2791.3 -0.6
2131     2797.4  
4261 2860.8   2860.1 0.6
4162     2919.3  
63     2957.1  
3142     2968.0  

Analysis of higher energy peaks is in progress.

Discussion

Dispersed fluorescence spectroscopy of H2CO has greatly increased knowledge of vibrational level structure in H2CO. Preliminary analysis indicates that similar results can be expected from the current work on D2CO. The increase in known vibrational levels will lead to a much better understanding of the formaldehyde PES.

Acknowledgments

We gratefully acknowledge stipends from the Dreyfus Foundation and the Hughes Foundation. This research has been sponsored by National Science Foundation grant CHE-9157713.