Instrumentation
Cavity Ringdown Laser Absorption
Spectroscopy ecause it is simple, fast, and verCavity ringdown satile, absorption spectroscopy is the method of choice for laser absorption performingoften noninvasive in situ measurements of concentrations and composition in spectroscopy with chemical systems. When performed under conditions where Beer's Law applies, it simple commercial yields accurate species concentrations in a straightforward manner. However direct pulsed lasers and absorption techniques typically lack the sensitivity of competing methods such as standard electronics laser-induced fluorescence multiphoton and photodepletion that are can be used to obtainionization based on detecting of the photon absorption event and this differultrasensitive ence limits their use in some trace eras analysis direct absorption as inThis reduced sensitivity results from the measurement of absorption as a small measurements change in the total transmitted source intensity. To achieve high sensitivity, flue-
J. B. Paul R. J . Saykally University of California—Berkeley
tuations in the source intensity must be minimized accordingly, which seemingly precludes the use of pulsed lasers for making direct absorption measurements of trace species. This is unfortunate, because modern commercial pulsed laser systems can be operated over much wider wavelength intervals than can continuouswave (CW) lasers, and they are generally easier to use (single-mode optical parametric oscillator/amplifier systems being a recent exception!) and less expensive. In this Report, we describe a promising new approach for making ultrasensitive direct absorption measurements with pulsed lasers. We call this technique, invented in 1988 by O'Keefe and Deacon (i), cavity ringdown laser absorption spectroscopy (CRLAS) (2). CRLAS combines very high sensitivity (fractional absorptions smaller than 1 ppm), good time resolution (~ 10 us), and perhaps most important a compelling degree of simplicity and generality. It has recently been used from the IR to the UV for the spectroscopy of metal clusters
Analytical Chemistry News & Features, May 1, 1997 287 2
Instrumentation
Figure 1 . A simple model for CRLAS.
and refractory molecules in pulsed molecular beams (3,4), for the study of free radicals in combustion systems (5-8) and in diamond film deposition (9), for bimolecular kinetics measurements (10-12), for the ssudy oo molecular ions in plasmas (13,14), foo the eetermination of absolute concentrations of small water clusters in supersonic expansions, and for the measurement of high overtones of molecular vibrations (15)) Basic principles Basic principles of the CRLAS experiment are shown in Figure 1. One can view in this simple model the light pulse produced by a tunable pulsed laser as a wavepacket of spatial extent given by the coherence length of the laser (here taken as C/2TCAV, where Av is the ffequency bandwidth of the pulse). The pulse is injected into an optical cavity formed by two highly reflecting mirrors, separated by a distance (L), which is chosen to be large with respect to the coherence length. Under these conditions the effects of multibeam interference that ordinarily define the familiar behavior of such optical cavities are
ence (i.e., frequency-dependent transmission) will be negligible, and the cavity will exhibit simple exponential decay of the light intensity. The cavityringdowntime is then independent of the transmitted laser intensity; combined with the long, effective pathlength, this accounts for the high sensitivity of the method. If an absorbing sample is placed inside the cavity under the above conditions, the cavity ringdown time will be shortened at those wavelengths where absorption occurs. The sample absorbance at a given wavelength is then very simply related to the cavity ringdown time by the equationA = tt/2(l/z - -/x0)) where A is the sample absorbance, t is the cavity round-trip time, x is the ringdown time, and T0 is the ringdown time insignificant, and the amount of energy of the empty cavity. coupled into the cavity is independent of the center frequency of the laser. One can In practice, the CRLAS experiment then simply view the laser pulse as a parti- consists of measuring theringdowntime cle, which will be partially transmitted into for each laser shot at each wavelength the cavity by the input mirror and will increment, averaging successive laser make many (typically several thousand) shots to reach a desired S/N. A plot of the round trips inside the cavity before its total cavity loss versus wavelength then intensity decays, which is caused by the yields the corresponding absorption specfinite losses sn the system. trum, wherein the spectral features of the sample are superimposed on the backA detector placed after the exit mirror ground cavity losses (x0 varies slowly with encounters a train of pulses (essentially wavelength). To achieve high absorption much less intense replicas of the original laser pulse) emerging from the cavity, sepa- sensitivity, the background losses must be small; that is, the mirrors must be very rated in time by the cavity round-trip ttme (2L/c) and wiih a ssmple exponential lecay highly reflecting. The relationship between mirror reflectivity absorption sensienvelope caused by the cavity losses. The tivity and the cavityringdowntime is time for the amplitude of this pulse train to given in Table 1 For verv trood mirrors decay to l/e of its initial value is called the available in the near-UV and visible re"ringdown time". The condition that the coherence length of the laser pulse be crirms (T? = QQ QQW^ ,ractional lasnrntirtn*; much less than the cavity mirror spacing is per pass as small as 5 x 1(T7 can be meaequivalent to requiring that the frequency sured with a sinrie laser pulse correbandwidth of the laser be much greater sponding to 10 000 passes through the than the cavity longitudinal mode spacing samnle and a measurement time of SO us If this condition is fulfilled using either defor a SO-cm cavity This capability is 'm scription the effects of multibeam interferpressive by any standard!
T a b l e 1 . R e l a t i o n s h i p b e t w e e n mirror r e f l e c t i v i t y , a b s o r p t i o n s e n s i t i v i t y , a n d c a v i t y r i n g d o w n t i m e . Mirror reflectivity
No. of passes toe - 1
Time constant to 1%
Time constant to 2%
0.9950 0.9990 0.9995 0.9999 0.99995 0.99999
100 500 1,000 5,000 10,000 50,000
±5.05 x10"5 1.1 x10~ 5.05 x 10~5 1.1 x 10" 6 5.05 x 10" 7 1.1 x 10" 7
± 1.02 2.04 1.02 2.04 1.02 2.04
288 A
Analytical Chemistry News & Features, May 1, 1997
x 10'4 x10 x10"5 x 10 x10"6 x10"7
Time constant to 3%
Time constant to 10%
±1.5x10"4 3.0 x 10' 5 1.5x10"5 3.0 x 10" 6 1.5x10~ 6 3.0 x 10" 7
±5.5x10"4 1.1 x 10" 5.5 x 1 0 ' 5 1.1 x 1 0 ~ 5 5.5 x 1 0 ' 6 1.1 x 10" 6
Design considerations One of the most appealing aspects of the CRLAS approach is the simplicity of design and operation. By considering just a few basic factors, one can easily construct a spectrometer capable of measuring fractional absorptions of just a few parts in 106. The most effective way to achieve high sensitivity levels is to use the highest reflectivity mirrors available, because higher reflectivity yields longer ringdown times which, in general increase the overall sensitivity of the apparatus (see Table 1). Short decay times place increasingly stringent requirements on the overall speed of the detector and associated electronics For a 0 5-m cavity length mirrors having R > 0 999 result in ringdown times long enough (> 2 us) to allow the 1 ISP of common 10- to 20-MHz detprtrtn; amplifier
