James R. Allkins 15485 One Oak Lane Monte Sereno, Calif. 95030
It has long been the dream of many spectroscopists to have available a high-intensity source of monochromatic radiation that could be tuned to any desired wavelength at will. Such a source is the tunable laser. It is not surprising, therefore, that in the nine short years since Sorokin and Lankard first observed stimulated coherent emission from a laserpumped organic dye (i ) and Schâfer et al. (2) realized the tunability of such a source, the tunable dye laser has experienced an extraordinary growth in its development (3-7). At present, owing in part to anticipated applications and in part to strong commercial competition, continuously tunable dye lasers are available at a cost that is well within the budget of most industrial laboratories and with an ease of operation that is comparable to that of fixed frequency gas lasers. Before becoming too engrossed, however, it should be recognized that the tunable dye laser is not the immediate answer to all problems, and despite its tremendous potential, it still has certain drawbacks. The first is that the effective tuning range is restricted to the visible region of the electromagnetic spectrum (typically 750-350 nm). Tunable ultraviolet radiation can be and is obtained by frequency doubling the visible radiation inside the laser cavity with a suitable nonlinear crystal (8), but even then the upper limit of the tuning range is usually confined to around 260 nm. Secondly, to cover the entire visible region entails the use of many different dyes; therefore, various dye solutions have to be switched in and out of the pumping area as each region is scanned. Tunable sources of coherent radiation other than dye lasers are available for other regions of the electromagnetic spectrum (9-12), particularly the infrared. Semiconductor diode lasers
and spin-flip Raman lasers emit in the "information-rich" infrared fingerprint region (4000-400 c m - 1 ) . Unfortunately, they too have their problems such as requiring cryogenic cooling and high-power magnetic fields to operate efficiently. They also cover a rather limited tuning range at any one time. However, their potential applications are so far reaching that these restrictions should be overcome in the very near future. Other tunable laser sources such as parametric oscillators and frequency mixing devices have, so far, been confined to the laboratory bench since they require a very stable temperature environment and very fine positioning controls. They also should achieve commercial viability once potential applications have been developed. In light of the above preamble, it is not surprising that most applications of interest to chemists with tunable laser sources have been performed with the tunable dye laser. The major part of this article will, therefore, be concerned with the impact of tunable dye lasers on analytical spectroscopy. In particular, applications which have taken full advantage of the laser's unique properties of high intensity, narrow spectral bandwidth, coherence, small beam diameter, and small divergence directionality will be discussed. The potential impact of other tunable sources will serve as a conclusion. Tunable Dye Laser
A simplified version of a typical dye laser is shown in Figure 1. Both the étalon for frequency narrowing and the nonlinear crystal for intracavity frequency doubling can be inserted in the laser cavity if and when required. To cover most of the near UV-VIS tuning range requires that a series of dyes in solution flow sequentially through the dye cell, since each dye only emits effectively over a limited range of some 20-50 nm. Alternative-
752 A · ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
ly, different dye cells containing different dyes can be swung in and out of the pumping cavity as required. Typical tuning elements and pumping sources are listed in Table I. Dyes found most effective for stimulated emission are the coumarins, rhodamines, cresyl violet, and fluorescein. Factors that could reduce the efficiency of the lasing process such as nonradiative transfers or dimer formation by the dye are minimized by the addition of quenchers and deaggregating agents to the dye solution. A comprehensive discussion of various factors that affect the efficiency of dye lasers has been published recently by Drexhage (13). Applications of Tunable Dye Lasers Before becoming too deeply involved in the various applications of tunable dye lasers in analytical specTable I. Typical Tuning Elements and Pumping Sources for Dye Laser Systems ( F l a s h l a m p a n d N 2 laser p u m p i n g is u s u a l l y b y w a y o f P2, w h e r e a s o t h e r lasers p u m p a x i a l l y a l o n g P1; see Figure 1 ) Pumping sources Tuning elements
Flashlamp N 2 laser Freq. dbld. ruby laser Freq. dbld. Nd: Y A G Freq. dbld. Ndiglass Another dye laser CW argon laser Xenon laser
Grating Prism/mirror assy Biréfringent filter Tilted étalon0 Wedged é t a l o n L y o t filter0 Acousto-optic filter Interferometer0
a Also used as frequency narrowing devices.
Report
troscopy, it appears appropriate to define more fully those qualities of the laser that make it unique from other, more conventional, sources of radiation. The primary property of the laser is that all of the available laser radiation is confined in a physically very narrow, low divergence, directional beam that can be placed precisely at any point in a required area. Furthermore, the beam possesses spatial coherence which means that it can be focused to a very small spot at any point along its path with a concomitant increase in power density. Not only is the beam narrow in size (spatially), but it is also narrow in frequency spread (spectrally). This means that most of the radiation intensity is contained in a narrow band of frequencies around a central dominant mode. Furthermore, this bandwidth of frequencies can be made even narrower by the insertion of suitable étalons into the laser cavity. The narrow bandwidth is extremely useful for high-resolution spectroscopy since source broadening limitations are almost completely removed. Also, the loss of energy accompanying frequency narrowing of a standard source by filtering through very narrow slits of a standard monochromator is not encountered with the laser, since almost all of the available energy of the laser is channeled into the narrow band of frequencies selected. Thus, we have a narrow beam of high-intensity radiation concentrated in a narrow band of frequencies that can be directed to almost any small area in a system without any substantial loss of power in getting there. Finally, there are the temporal properties of a laser, that is, the variations of output power with time. Tun-
able lasers fall into two broad classes, namely pulsed or CW (continuous wave). With pulsed dye lasers, i.e., those pumped by a pulsed excitation source, the available energy emerging from the laser is confined in a stream of short bursts or pulses. Pulse characteristics are generally determined by the temporal characteristics of the pumping source. With pulsed nitrogen laser pumped dye lasers, the pulse lengths tend to be short (nsec), whereas the peak powers of each pulse tend to be low. With flashlamp pumping the pulses tend to be longer (μββο), but the peak powers tend to be high. Flashlamp pumping, therefore, gives more energy per pulse, but nitrogen lasers have faster repetition rates (more pulses per sec). Pulsed lasers are used to great advantage in studies of time-related phenomena such as lifetimes, relaxation rates, reaction ki netics, or in studies involving nonlin ear optical phenomena in which high peak powers are required. Pulsed la sers may be operated in either a single pulse mode or repetitively in which a stream of pulses is generated with a regular time interval between each pulse. Continuous wave (CW) lasers are pumped by CW sources such as a CW argon ion laser. Although the power output of a CW laser cannot attain the peak power of a pulsed laser, the CW output is continuous and therefore more akin to the conventional sources of radiation used in most spectroscop ic techniques. The average power out put of a C W laser is usually higher than that of a pulsed laser, but, more importantly, the power stability of a CW source is usually much greater. The power output from a pulsed laser can vary considerably from pulse to
Figure 1. Schematic of typical dye laser Solutions of dyes in dye cell D are pumped by ex ternal sources Pi or P2 to produce stimulated emission. Generated emission is wavelength tuned by tuning element Τ and frequency nar rowed by optional étalon Ε. Nonlinear crystal X is inserted into cavity when tunable radiation is re quired in ultraviolet. A small fraction of tuned ra diation is transmitted by output mirror M, the re mainder being reflected for further optical amplifi cation. An iris I is often inserted in cavity to con trol mode characteristics of intracavity beam
pulse. This makes the CW laser espe cially useful in applications which de mand a steady source of very stable power contained in a very narrow bandwidth of frequencies. The following applications exemp lify one or more areas where the prop erties of a tunable laser have proven superior, or at least equivalent, to those of previously used conventional sources. Advantages found with the laser will include greater sensitivity to improve limits of detection, greater resolution to reveal more information content, and better selectivity to aid in identification. Absorption and Fluorescence Spectroscopy Two of the more interesting proper ties of a tunable laser are its high spectral brightness and narrow fre quency bandwidth. This implies that both high-sensitivity and high-resolu tion spectroscopy should be natural candidates for tunable laser applica tions. The sodium atom has received particular attention in this respect, since two of its primary atomic ab sorption lines (E>i and D 2 ) lie close in wavelength to the emission peak of one of the more efficient lasing dyes, namely, rhodamine 6G. To demonstrate the potential detec-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975 · 753 A
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Figure 2. High-resolution spectrum showing hyperf ine structure of sodium D 2 line recorded with CW dye laser Lower part shows theoretical structure. F and F denote quantum numbers of total angular momentum of hyperfine levels of 32P3/2 and 2 Si/2 levels, respectively Figure taken from ref. 15 and reproduced with permission of Springer-Verlag, New York
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tion sensitivity of a system involving a dye laser as its source, Jennings and Keller (14) observed the laser-excited fluorescence emission from a sample of sodium contained as a vapor in a simple heated cell. Using a frequency narrowed (~0.003 nm) CW dye laser with 50-mW output power at 589.6 nm as the source, they were able to detect as little as 0.016 Χ 1 0 - 1 5 gram (4.2 X 10 5 atoms) of sodium vapor. Detection was by simple visual observation of the fluorescence emission. Their sys tem has since been modified to in crease the sensitivity by a factor of 200 (5), which is an improvement in sensi tivity of approximately 106 over sodi um detection by vapor lamp excita tion. In the area of high-resolution spec troscopy, considerable interest has been generated in the past two years since this is one area in which the laser can show dramatic improvements over conventional techniques. As the line-
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width of the laser can be frequency narrowed to a value of approximately Yiooth of that normally observed for a single atomic absorption line, hyper fine structure can be easily resolved with a laser source. To obtain highresolution spectra requires a source with small bandwidth, excellent fre quency stability, and good stable tunability. The frequency stability must be appreciably better than the desired resolution, and the laser must be able to tune smoothly without mode hop ping. Figure 2 illustrates the high-res olution capabilities of a frequency sta bilized CW dye laser as recorded by Hartig and Walther (15) for the sodi um D2 line. Demonstrated resolution was of the order of 3.5 X 10~ 5 nm (30 MHz) [this has since been improved to 10 MHz (6)], and the frequency of the laser was stabilized to ±1.5 MHz. Hy perfine structure is clearly resolved. To observe such hyperfine structure requires not only that the source be
narrow, but also that line broadening of the sample be reduced to a minimum. The two primary causes of spectral line broadening in the visible are collision broadening and Doppler broadening. Collision broadening may be reduced by lowering the pressure in the sample cell. The high spectral brightness of the laser can more than compensate for the corresponding reduction in atom density. Doppler broadening, arising from axial motion of the sample atoms in the sampling beam, can be reduced by such techniques as saturation spectroscopy (16), by confining the sample in an atomic beam (25), or by the relatively new technique of two-photon absorption spectroscopy (17). Using a special jet stream dye-flow system and a CW argon laser pump, Wu et al. (18) have recently recorded the hyperfine structure of I2 with a resolution of one part in 109. The iodine sample was contained in a molecular beam, and the laser stabilized to six parts in 10 13 . On a more practical note, Gibson and Sandford have taken advantage of the sodium doublet:rhodamine 6G intensity match to remotely monitor sodium concentrations in the atmosphere. They were particularly interested in variances of the naturally occurring sodium layer found at an altitude of about 90 km. Results have been obtained for both day (19) and nighttime (20) concentrations. Other gaseous constituents of the atmosphere including NO2, S 0 2 , and I 2 have also been detected remotely using tunable dye laser sources (21, 22), as have algae concentrations in the sea (23). On the laboratory scale, one of the most widely reported uses of dye lasers in analytical spectroscopy has been in the field of laser-excited atomic fluorescence flame spectrometry (AFFS), a technique for analyzing trace elements (24-27). To detect small atom concentrations in a flame requires good signal-to-noise ratios for the atomic fluorescence emissions. To obtain strong fluorescence emission requires an excitation source of high spectral brightness that is also monochromatic. The source should also be stable when repetitively pulsed, have a high peak power output, and a small duty cycle (small ratio of on-to-off time). Such a source is the pulsed dye laser. The tunability of the dye laser gives the added advantage that one source may be used to study many different atoms in a sample, and a variety of atoms have been studied by this technique. A typical system is shown in Figure 3, in which gated detection is employed to remove all noise except that generated by scattering of the laser ra-
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Figure 3. Block diagram of experimental system for laser-excited atomic fluorescence flame spectrometry Figure taken from ref. 25
diation within the flame. Background scatter can be reduced even more by the use of suitable nebulizers or by recording fluorescence emissions from nonresonance transitions. It would appear that with such an ideal source as the pulsed dye laser for AFFS, lower limits of detection and wider range, linear analytical curves would result from improved signalto-noise ratios when compared with conventional trace analytical methods such as atomic absorption, atomic emission, or atomic flame fluorescence excited by conventional sources. Extensive results (28), however, show that at best the detection limits for laser-excited flame fluorescence are only slightly better than those of other atomic flame spectrometric techniques. The reason for this anomaly has been traced in part to a saturation effect (29, 30). In this effect, the intensity of the laser irradiance reaches such a level that the atomic absorption transition becomes essentially saturated. Any further increase in laser irradiance simply results in increased background scattering. A limit is therefore set on the linear relationship between the fluorescence signal and the exciting laser radiation intensity. In more exact terms, the atomic fluorescence radiance assumes a nonlinear dependence on the exciting spectral radiance once the excitation radiance reaches a certain level. By irradiating only certain areas of the flame and by careful focusing of the laser beam to a power level approaching saturation, Omenetto and his coworkers (31 ) were able to extend the linear range of certain analytical curves by a factor of 1000. One other advantage observed when working near saturation is that the atomic fluorescence emission signal is virtually independent of source fluctuations and is also less affected by collision quenching. Further improvements in detection limits are expected when laser tuning ranges are finally extended further into the UV since many of the more sensitive absorption lines (i.e., those with high ab-
sorption oscillator strengths) are in the mid to far UV region. Initial results obtained in the ultraviolet region for Mg, Ni, and Pb have already been reported by Kuhl and Spitschan (32) using a frequency doubled dye laser as the excitation source. Other recent applications involving tunable laser excitation of flame fluorescence have been in the detection and measurement of CH (33) and C2 (34) radicals in flames. Flameless atomic fluorescence spectroscopy with laser excitation has also been demonstrated recently by Neumann and Kriese (35) using a frequency doubled flashlamp pumped dye laser. They were able to detect a low limit of 0.2 pg of Pb and extend the linear range of the Pb analytical curve by two orders of magnitude over that obtained with more conventional sources. Finally, the first report has recently appeared on the analytical applications of laser excited fluorimetry of molecules in the condensed phase (36). Another technique that has received considerable attention in the past few years is laser enhanced absorption spectroscopy (37) or selective intracavity laser quenching, as it is some-
times called. In this technique, a sample is introduced into the cavity of a dye laser operating under broadband conditions, as shown in Figure 4. Broadband conditions are obtained by replacing the tuning element in the cavity by a broadband totally reflecting mirror. All wavelengths of the stimulated dye are then emitted simultaneously without spectral narrowing. If the sample has an absorption or absorptions at wavelengths within the broadband emission curve, losses will occur in the laser cavity at these wavelengths. The result is that the usually smooth emission profile from the dye laser now contains absorption bands definitive of the sample. A rather striking example is shown in Figure 5 in which a sample of NO2 has been introduced into the cavity of a flashlamp pumped dye laser (38). This method is particularly sensitive since it involves a selective spoiling of the gain inside a laser cavity. Single pass gain in a cavity is critical for lasing to occur, especially near threshold. A further advantage of the method is that the whole spectrum is obtained in one shot, as it were, since
Figure 4. Dye laser system set up for broadband operation. Laser enhanced absorption spectrum of intracavity sample is recorded by spectrograph
756 A · ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
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