Cavity Ring Down Spectroscopy

Introduction

Cavity Ring-Down Spectroscopy (CRDS) is a relatively new (1988) and highly sensitive absorption spectroscopy technique which has found a wide range of uses since its inception. CRDS measures the rate of absorption of a light pulse confined in a stable optical cavity formed by two highly reflective mirrors (reflectivity, R > 99.9%). A short laser pulse coupled into the cavity is reflected back and forth and, every time the light is reflected, a small fraction (1 - R) 'leaks out', leading to exponential decay of the pulse in the cavity. This light is detected by a photomultiplier tube (PMT) and can be recorded using an oscilloscope and PC. The decay time (ring-down time) of the cavity is determined by measuring the time dependence of the intensity of light leaking out of the cavity.

Light absorbed by the sample between the mirrors will increase the decay of light and hence reduce the ring-down time. The molecular absorption coefficient, a, is directly linked to the ring-down time, thus, absorption spectra can be recorded.

A cavity ring-down experiment has been constructed to study molecules in a supersonically cooled molecular beam.
See the photos section for current progress.

vacuum chamber
laser system
test measurements
current investigations

The Experiment

In our experiment the ring-down cavity is in a vacuum chamber, with the mirrors mounted on adjustable bellows at a separation of 770 mm. The molecule being studied can be seeded into argon or helium (at a range of pressures) and expanded into the vacuum chamber through a pulsed nozzle to intersect the cavity axis. A fast ionisation gauge is mounted on a linear feedthrough perpendicular to the nozzle and cavity axes to optimise the timing and shape of the gas pulse. The chamber is pumped by a diffusion pump backed by a rotary pump.

T he laser and detection system for our CRDS set-up is shown below.
A Nd:YAG laser (Continuum, Surelight I-10) is used to pump a pulsed dye laser (Sirah, Cobra Stretch). A spatial filter [telescope] is used to ensure the laser had a Gaussian profile. The spatial filter was used with a lens in front of the cavity to couple the light into the TEM₀₀ mode of the cavity.

Our experimental set-up

The light exiting the cavity was detected by a photomultiplier tube (PMT) coupled to an oscilliscope which was connected to a PC. The PC is used for experiment control and data acquisition via a LabVIEW program.

Test measurements have been made using atmospheric oxygen (bulk) and iodine (molecular beam). In the I₂ molecular beam a rotational temperature of less than 10 K and a vibrational temp of approximately 150 K was easily achievable. A sample Iodine spectra is shown below.

The experiment is now being used to study the torsional vibrational modes of 1,4-bis(phenylethynyl)benzene.

1,4-bis(phenylethynyl)benzene

Links

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Bristol Laser Group CRDS page a very good introduction to CRDS.

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