Cavity Ringdown Spectroscopy of Acrolein

Introduction

Acrolein is a prototypical conjugated bond system which absorbs in the near ultraviolet.

Since acrolein does not fluoresce, Cavity Ringdown (CRD) spectroscopy is used to measure the absorption spectrum of acrolein in a molecular beam.

Cavity Ringdown (CRD) Spectroscopy

A pulse of laser light is directed into an optical cavity which has highly reflective mirrors on each end. The light bounces back and forth through the sample thousands of times. The small intensity of light that escapes through the second mirror is monitored by a photomultiplier tube (PMT). The ringdown time of the cavity is a measure of absorbance by sample contained between the end mirrors. The large optical pathlength results in a highly sensitive absorption technique.

Experimental Setup

A Nd:YAG laser emits 532 nm light pumping a dye laser, which is tuned over a wavelength range of 680-780nm. The red light passes through a doubling crystal which converts it into ultraviolet (UV) light. The beam translation introduced by the angle-tuned doubling crystal is counteracted with a quartz compensating block. The mixture of UV and residual red light is separated by a pair of 60° prisms, and a second pair of prisms compensates for the beam translation. Some of the red light is directed into a hollow cathode lamp for calibration. The pure UV is directed through a spatial filter, which consists of a focusing lens, pinhole, and collimating lens. The high quality light is sent to the molecular beam chamber where it interacts with the sample. The signal detected by the PMT is averaged by the oscilloscope and sent to the computer, where the lifetime is fit to an exponential decay to monitor absorbance.

Results

The room temperature spectrum of acrolein is very congested with overlapping peaks on a steeply sloped baseline. The flat baseline in the molecular beam spectrum is an indication that the acrolein has been cooled, simplifying the spectrum.

The origin region of the S₀ -> S₁ electronic transition is a band with partially resolved rotational structure.

Discussion

The preliminary assignments for the molecular beam spectrum include nine fundamental and two combination vibrational bands. The MOPAC theoretical calculations generally agree with the lower frequency vibrations and overestimate the higher frequency vibrations.

Acknowledgments

We gratefully acknowledge Dr. Marsha Lester (University of Pennsylvania) and Dr. Kevin Lehmann (Princeton University) for useful discussions. We also thank Brad Mulder for fabrication of the apparatus. This research has been sponsored by National Science Foundation grant CHE-9157713.