Closed-loop intracavity photoacoustic overtone spectroscopy

Vibrational Spectroscopy, 1 (1991) 377-381 Elsevier Science Publishers B.V., Amsterdam

377

Closed-loop intracavity photoacoustic overtone spectroscopy Christopher R. Moylan IBM AI&en

Research Center, San Jose, CA 95120 (U.S.A.) (Received 22nd October 1990)

Abstract A technique is described whereby photoacoustic spectra of sample gases may be taken without the need for subsequent calibration of the wavelength scale. Laser power is measured externally with a pyroelectric detector, rather than internally with a silicon photodiode. Previously observed kaloning effects are dramatically reduced. Recent improvements in photoacoustic cell design by Zare et al. are employed. The technique is demonstrated by measuring the fifth C-H stretching overtone spectrum of diethyl ether. Keywords: Photoacoustic spectroscopy; Diethyl ether; Overtone spectroscopy

Overtone spectroscopy and photochemistry have received increasing attention recently because of the understanding they provide about vibrationally excited molecules, and because of experimental indications that such molecules can react in non-statistical or even desired ways [l-4]. High overtone absorption spectra are therefore of significant importance, but the cross-sections for absorption are small [5] (of the order of 1O-25 cm*). Taking overtone spectra can be tedious. This paper describes a modification of the traditional method that, when combined with various improvements made by previous workers, decreases the tedium and allows a high degree of automation to be employed. EXPERIMENTAL

The most substantial difference between these experiments and the standard methods is closedloop operation. The spectrum consists of a series of points, and the wavelength of each point is measured experimentally at the time rather than being assigned after a separate calibration step 0924-2031/91/$03

SO

@ 1991 - Elsevier Science Publisher! i B.V.

independent of the data scan. Indirect measurements such as calibrations cannot be as accurate as direct measurements, and this problem has caused errors in wavelength assignments for photoacoustic overtone spectroscopy [6]. Consequently, it was decided to measure every wavelength directly rather than calibrate, even if scan speed were reduced as a result. Wavelengths were measured in this work with a wavemeter in the low-resolution mode [7], which provides six significant figures for each measurement. The wavemeter can read only a continuous-wave (cw) beam, but the beam must of course be chopped to measure the photoacoustic signal. Obviously, both wavelength measurement and signal detection cannot be performed simultaneously, but this conflict does not preclude closed-loop operation, as some have concluded [S]. The system must simply alternate between chopped and cw modes. The switching was accomplished here by mounting the chopper wheel on the forcer of a linear stepper motor. The chopper was moved back and forth on instructions from the IBM PC-AT that acts as system controller.

C.R. MOYLAN

The switching between cw and chopped lasing modes does introduce additional delays, because one must wait for the chopped signals to reach full value each time the chopper is moved into the beam, but this can be done completely automatically. The user need not be present during scans except occasionally, and the separate calibration step is eliminated, as mentioned above. A second aspect of this work that is different from earlier studies is the manner in which intracavity power is measured. The microphone signal must be divided by the intracavity laser power to correct for the large variations in laser intensity observed as one tunes through a dye curve. The standard device used for laser power measurement in photoacoustic spectroscopy has been the silicon photodiode. But silicon photodiodes themselves have wavelength-dependent responses, so the calibration is only approximate unless the wavelength range of a spectrum is small. In this work, a pyroelectric detector was used. Although the pyroelectric detector output is strongly dependent on chopping frequency, it is not dependent on wavelength. Because each experiment was performed at constant chopping frequency but varying wavelength, such a detector provides a true calibration factor for the laser intensity. The intracavity power was measured externally in this work, as has been done by others previously [9], in order to alleviate aggravating “etaloning effects” that have been seen both in this laboratory and by other workers [lO,ll] when the standard technique of monitoring the light reflected off a cell window has been used. The transmission spectra of several output couplers were measured over their respective wavelength ranges with a Hewlett-Packard Model 8452A diode-array spectrophotometer and fitted to sixthorder polynomials. Given a measured external power and a wavelength, the relative intracavity power can be calculated. The remaining aspects of the method used here have all been featured in earlier work, but not necessarily together. The light source used was a ring dye laser [7,8]. A ring laser, by virtue of its traveling wave cavity, has no nodes in the dye jet and thus exhibits greater pumping efficiency and higher intracavity power than standing-wave dye

lasers. The laser was used with conventional optics; the output coupler was not replaced with a total reflector as is often done [12,13] to increase the intracavity power further. This sacrifice in intracavity power was made so as to measure both the power and the wavelength externally, the latter making closed-loop operation possible. The geometry of the ring laser allows a photoacoustic cell to be mounted between the upper fold mirror and the optical diode without any modification of the laser components. On many occasions, installation of the cell consisted simply of peaking the laser, mounting the cell and adjusting only the output coupler or upper fold mirror vertical control to achieve lasing. The photoacoustic cell was designed to maximize the signal-to-noise ratio using the well established features of Brewster angle windows and baffles [12,14] in addition to more recent innovations by Davidsson et al. [ 131 that include dual microphones with summed output, modular design using Cajon Ultra-Torr fittings, a drying agent reservoir to remove peaks due to water and the use of xenon as a buffer gas so that the cell becomes resonant at accessible chopping frequencies. Typically, the cell contained < 100 Torr of the gas under study and sufficient xenon to bring the total pressure to 300 Torr. These pressures produced resonant photoacoustic cell behavior at chopping frequencies of 1300-1400 Hz. The resonant frequency for any given longitudinal mode in a cell is proportional to the speed of sound in the gas mixture. The Newtonian speed of sound in a fluid, a, is given by a = ,/m, where P is the pressure and p is the density. Therefore, the resonant chopping frequency is a sensitive function of the composition of the gas mixture in the cell, and is highly variable. This variability has convinced some workers [15] to operate in the non-resonant mode at all times rather than attempt to modify cells to make them resonant in each instance. But there is no need to modify the cell to bring it into resonance with a given gas mixture; one merely tunes the laser to a wavelength that is known or suspected to be absorbed by the sample, and adjusts the chopping frequency to maximize the signal. Changing the chopping frequency does indeed alter the voltage output per

CLOSED-LOOP

INTRACAVITY

PHOTOACOUST’IC

OVERTONE

SPECTROSCOPY

detector (Eltec 404-6) is sent by the detector controller (Eltec 610) to a single-phase lock-in amplifier (Stanford Research Systems SR 510). The sensitivity and the phase of the lock-m are adjusted for the power measurement at that wavelength, and are left constant for the duration of the experiment. Separate experiments have shown that the phase of the power signal does not vary with wavelength. After the single-phase lock-in parameters have been set, the chopper is moved out of the beam and the wavelength moved to the first point of the scan. The chopper moves back into the beam, and data collection starts. The outputs of the two microphones (Knowles EK 3024) are simply connected together and sent to a preamplifier (Stanford Research Systems SR 550) and from there to a two-phase lock-in amplifier (Stanford Research Systems SR 530) operated in the r,B mode. The sensitivity of the lock-in is increased in stepwise fashion, waiting for signal recovery each time, until an overload occurs. Then the sensitivity is backed off one step and the signal is read at that level. The power is read off the other lock-in, and data collection for that point is complete. The chopper moves out of the beam, and the rotary stepper motor adjusts the birefringent filter until

watt of the pyroelectric detector, but this effect manifests itself only in differences in the absolute value of the calculated spectral intensities, which are arbitrary units anyway. If one desires to produce actual cross-section numbers for the spectral intensities, performing all experiments at a single non-resonant frequency would be an option, although calibrating the spectrum by including a known mole fraction of a standard such as methane [5] or ethylene [16] would be a simpler alternative. The overall setup is shown in Fig. 1. Once lasing has been achieved with the cell mounted in the laser cavity (Coherent 699-01) and the chopping frequency has been adjusted to maxi&e the photoacoustic signal, the automated routine can begin. The rotary stepper motor (Compumotor A 57-51 controlled by a Compumotor 2100-2-488 indexer) rotates the dye laser birefringent filter by means of a Newport RC-4 remote control cable until the half-way point of the proposed wavelength scan is reached according to the wavemeter (Burleigh WA-lo). Then the chopper (Stanford Research Systems SR 540) is moved into the argon ion laser beam (Coherent Innova 200~15/3) by means of the linear stepper motor (Compumotor LX-L3C-P15). The signal from the pyroelectric I

I

I

PC-AT IEEE-488

Lock-In

Amplifier

I RS-232

3

379

Interface

Interface

Dual-Phase Lock-In Amplifier

Cable

Fig. 1. Closed-loop intracavity photoacoustic spectroscopy apparatus.

380

the wavemeter shows that the next point has been reached. This process repeats until the entire spectrum has been obtained, The only action required by the investigator is occasional peaking of the dye laser alignment if the entire dye tuning curve is being traversed. The two lock-in amplifiers, the wavemeter and the rotary stepper motor are all controlled over an IEEE-448 interface. The linear stepper motor (a combination indexer and driver) can be addressed only over an RS-232 interface. Better linear stepper performance would be obtained with the more expensive combination of 2100-2-488 indexer and L-L3C-P15 driver. When changing wavelengths, a criterion must be set for when the laser is “at” the desired wavelength. The data acquisition program used here establishes a window within which the desired wavelength is considered to have been reached. The velocity and acceleration of the rotary stepper motor are also functions of the window size, so that the motor does not cause the laser to overshoot the desired wavelength or oscillate around it. The time it takes to do an experiment is a function of several parameters: the number of points, the lock-in time constant, the number of time constants of delay taken each time the sensitivity of the lock-in changes or the chopper moves into the beam and the age of the dye mixture. The last parameter affects the experimental timing because when the dye mixture is old or has become heated during several hours of operation, the line width of the dye laser fluctuates enough that the wavemeter often takes many seconds to read the wavelength rather than only ca. 1 s. Under these circumstances, changing wavelengths rather than signal measurement becomes the rate-limiting step in the process. With a fresh, cool dye mixture, a lock-in time constant of 1 s and five time constants of delay, the system takes 100 points per hour.

SAMPLE RESULT

As an example of a spectrum taken by the method described above, 100 Torr of diethyl ether

C.R. MOYLAN

I

1

I

1

I

,

,

,

,

4 14750

15030

15310 Photon

15590 energy (cm-’

15870

16150

)

Fig. 2. Microphone signal as a function of photon energy for a scan of 100 Torr of diethyl ether vapor in 200 Torr of xenon.

vapor and 200 Torr of xenon gas were put in the cell and the spectrum was run over most of the DCM dye tuning curve. In this wavelength range, the 0 + 6 overtone transitions of the C-H stretching vibrations could both be seen (or, rather, heard, as this is photoacoustic spectroscopy). Points were taken every 3 cm-l with a lock-time constant of 1 s and a chopping frequency of 1318 Hz. Figure 2 shows the microphone signal as a function of wavelength, Fig. 3 shows the laser power as a function of wavelength and Fig. 4 shows the spectrum obtained by dividing the values in Fig. 2 by those in Fig. 3. The frequencies at which the dye laser was realigned, 14970, 15 430, 15 570 and 15 970 cm-l, are easily seen in Fig. 3. Mode-hopping behavior and Ctaloning effects in the dye

I

14750

1

I

1

15310 Photon

I

1

I

15590 energy (cm”

1

15870

I

16750

)

Fig. 3. Intracavity laser power signal determined from external power measurement and the transmission spectrum of the laser’s output coupler.

CLOSED-LOOP

INTRACAVITY

PHOTOACOUSTIC

OVERTONE

SPECTROSCOPY

entire spectrum may be taken automatically. Better intracavity power measurements can be obtained by using a pyroelectric detector rather than a silicon photodiode and by measuring the power externally as others have suggested.

15030

15310 Photon

15590 energy

(cm-‘)

Fig. 4. Fifth C-H overtone spectrum of 100 Torr of diethyl ether as determined. from the data in Figs. 2 and 3.

laser cavity are clearly visible in both Figs. 2 and 3, but they canceled each other out almost exactly, so that the effects barely appeared in the final spectrum in Fig. 4. It is therefore concl uded that the microphone signal was linear with intracavity laser power at any given wavelength. Note that even when the laser was detuned (as at 15 570 cm-l ) the ratio of the microphone signal to the laser power signal remained unaffected. Only . at the high-frequency end of the tuning curve, where the laser started to flicker, does significant noise appear in Fig. 4. Despite the lack of data smoothing, the spectrum shown in Fig. 4 compares quite well with that taken by Fang et al. [17]. The methyl and methylene peaks appeared in this work at 15 931 and 15020 cm-‘, respectively. The corresponding peaks as measured by Fang et al. appeared at 15 950 and 15 060 cm- ‘. The discrepancies can presumably be accounted for by the calibration uncertainties that they mentioned [6]. Conclusions Intracavity photoacoustic overtone spectroscopy can be carried out conveniently in closed-loop fashion, with measurement A’ the laser wavelength at each point rather than calibration of the birefringent filter and in.terpolation. By mounting the optical chopper on a linear stepper motor, the

The author thanks Jonathan H. Gutow and Professor Richard N. Zare of Stanford University and Dr. James S. Wong of IBM for many helpful discussions, assistance in procuring photoacoustic cell components and reprints/preprints of their work. The author also thanks Dr. Joseph M. Jasinski of IBM for helpful discussions and Michael R. Swanson of Dartmouth College for carrying out improvements to the data acquisition program.

REFERENCES 1 K.V. Reddy and M.J. Berry, Chem. Phys. Lett., 66 (1979) 223. 2 A. Schwebel, M. Brestel and A. Yogev, Chem. Phys. Lett., 107 (1984) 579. 3 J.H. Gutow, D. IUenerman and R.N. Zare, J. Phys. Chem., 92 (1988) 172. 4 A. Sinha, M.C. Hsiao and F.F. Crim, J. Chem. Phys., 92 (1990) 6333. 5 J.S. Wong and C.B. Moore, J. Chem. Phys., 77 (1982) 603. 6 H.L. Fang, R.L. Swofford, M. McDevitt and A.B. Anderson, J. Phys. Chem., 89 (1985) 225. 7 J.M. Jasinski, Chem. Phys. Lett., 109 (1984) 462. 8 C. Douketis and J.P. Reilly, J. Chem. Phys., 91 (1989) 5239. 9 H.L. Fang, D.M. Meister and R.L. Swofford, J. Phys. Chem., 88 (1984) 405. 10 R.G. Bray, W. Her&e, SK. Liu, K.V. Reddy and M.J. Berry, Chem. Phys. Lett., 47 (1977) 213. 11 M.W. Crofton, C.G. Stevens, D. Klenerman, J.H. Gutow and R.N. Zare, J. Chem. Phys., 89 (1988) 7100. 12 C. Manzaneres I., N.L.S. Yamasaki, E. Weitz and J.T. Knudtson, Chem. Phys. Lett., 117 (1985) 477. 13 J. Davidsson, J.H. Gutow and R.N. Zare, J. Phys. Chem., 94 (1990) 4069. 14 A.C. Tam, Rev. Mod. Phys., 58 (1986) 381. 15 B.R. Henry and M.G. Sowa, Prog. Anal Spectrosc., 12 (1989) 349. 16 C. Manzaneres I., N.L.S. Yamasaki and E. Weitz, J. Phys. Chem., 93 (1989) 4733. 17 H.L. Fang, D.M. Meister and R.L. Swofford, J. Phys. Chem., 88 (1984) 410.