INSTRUMENTATION
ULTRAFAST SPECTROSCOPY Mary J. Wirth Department of Chemistry & Biochemistry University of Delaware Newark, DE 19716
Time-resolved spectroscopy has been an active research area in analytical chemistry for more than 30 years. During this period, technological developments have improved time resolution from the millisecond to the subpicosecond range. Measurement of t h e temporal behavior of chemical systems has provided a new basis for selectivity. New technology has also allowed analytical spectroscopy to evolve beyond traditional boundaries to include the characterization of structurally heterogeneous systems such as surfaces and thin films, biological systems, electronic devices, and advanced materials. Time-resolved spectroscopy is now being used to provide structural and dynamic information to solve new types of chemical measurement problems and to improve qualitative and quantitative analyses. Ultrafast spectroscopy is an interdisciplinary field that spans physics, chemistry, biology, and electrical engineering in the study of the fundamental nature of the myriad of relaxation processes. The field is an exciting one in which researchers pursue advances in short-pulse generation and applications. Recent applications in chemical physics (2), biology (2), and electrical engineering (3) have been reviewed, as
have methods of short-pulse generation (4). In even-numbered years, the Optical Society of America sponsors conferences on ultrafast phenomena, and Springer publishes these proceedings as a book series. Ultrafast spectroscopy is receiving increasing attention from the analytical chemistry community, although it is far from being commonplace. The ultrafast time scale extends approximately from 10 fs (10" 14 s) to 100 ps (10~10 s). The fast end of this range is marked by the limit of laser technology and the slow end by photomultiplier technology. Ultrafast techniques require multiple-beam optical methods for detection. The need for high sensitivity and the desire for general applicability and convenience have combined to limit much of analytical spectroscopy to the time scale of photomultiplier technology. Despite these disadvantages, analytical spectroscopists are devoting more time to working in the experimentally difficult area of ultrafast spectroscopy. Lifetime measurements using photomultipliers In 1957 Keirs, Britt, and Wentworth published the first analytical paper on time-resolved spectroscopy (5). At that time phosphorescence was known to originate from a transition between specific molecular states (6), and these workers expected that the phosphorescence lifetime, r p , would be unique for a given molecule. They demonstrated
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that T P could be used as a parameter for chemical identification and for mixture analysis. The apparatus consisted of a pair of slotted wheels, one on each side of the sample, to mask the sample alternately from the arc lamp and the detector. By controlling the relative angular positions of the two slots, the emission was observed at a known delay time from the excitation. Their time resolution of 1 ms was limited by the sizes of the slots in the wheels and by the motor speed. Time-resolved spectroscopy thus began in the mechanical era. The results of this first paper bear striking similarities to and differences from current work. Today much analytical research involves the same goal of using lifetimes for molecular identification. However, the time resolution has improved by 7 orders of magnitude, from 1 ms to 0.1 ns. Electronics and quantum electronics have replaced mechanical parts, due to the invention of the laser and mode locking; the development of repetitively pulsed lasers; and the availability of faster photomultipliers, particularly the microchannelplate photomultiplier. Improvements in optical technology and continual advances in digital electronics and laboratory computers have made nanosecond fluorescence experiments routine. These improvements in speed have permitted the study of fluorescence rather than phosphorescence. The use of phosphorescence is restricted to very viscous or solid media, whereas fluores0003-2700/90/0362-270A/$02.50/0 © 1990 American Chemical Society
cence is emitted by a wide variety of systems under less restrictive conditions. Faster time resolution was thus essential in making the original idea generally applicable. Fluorescence lifetimes can be determined rapidly enough to be implemented in chromatographic detection (7). Early efforts in time-resolved fluorescence spectroscopy involved the measurement of fluorescence decay times of laser dyes. With the many improvements in optical technology, fluorescence lifetime measurements have become generally applicable. Impressive advances have been made in electronics at MHz and GHz frequencies, accelerating the development of frequency-domain techniques (8). The principle of frequency-domain techniques is that a decay function in the time domain can equivalently be determined in the frequency domain. By modulating the light beam at a variety of frequencies and measuring the amplitude demodulation and phase shift of the emission, the time decay is calculated from the simple Fourier transform relations. Deconvolution of the excitation pulse is intrinsic to the frequency-domain measurement and extends the effective time resolution of photomultiplier detection by about a factor of 10. Lifetime measurements by frequency-domain techniques are also very rapid and versatile. Multiplecomponent samples can be analyzed without prior knowledge of the lifetimes on an analytically useful time frame (9,10).
time scale, the intensity depends on which polarization is measured. This phenomenon, fluorescence anisotropy, results from the preferential excitation of solutes oriented with respect to the polarization of the laser beam. The decay of the anisotropy as a function of time is attributable to molecular reorientation. In part c, on the 1-ps time scale, the intensity increases with time. This represents the evolution of the
Ultrafast relaxation processes
Now that the analytical utility of lifetime measurements has been vastly improved by the advancement from the millisecond to the nanosecond time scale, what can we expect from the advancement through the next 4 or 5 orders of magnitude? The technological barriers to measuring ultrafast processes can inhibit one's imagination about possible future applications. But suppose we had a photomultiplier with a rise time of 1 fs, along with a tunable laser with output pulses of 1 fs. Given this equipment, most spectroscopists would immediately go into the lab, put their most interesting fluorescent sample into the ultrafast fluorometer (perhaps one whose decay they have deconvoluted), and find out what the decay really looks like. Figure 1 gives an educated guess as to what might be observed in a hypothetical femtosecond emission experiment, showing the decay plotted on different time scales. In part a, on the 10ns time scale, the familiar fluorescence decay is shown. In part b, on the 100-ps
Figure 1. Emission behavior on different time scales. (a) Fluorescence decay; (b) fluorescence anisotropy; (c) increase in fluorescence as the spectrum evolves; and (d) resonance Raman decay, with background fluorescence.
Stokes shift as the environment of the solute adjusts from ground-state solvation equilibrium to excited-state equilibrium (11). As the time scale becomes increasingly shorter, other unfamiliar features of the fluorescence spectrum may be observed. One is unrelaxed fluorescence. Normally, fluorescence emission arises from a transition between the lowest excited electronic state to the ground state. Collisions with the environment cause extremely fast internal conversion, resulting in the same emission spectrum regardless of which wavelength is used for excitation. Fluorescence from vibronic states might be observed because vibrational relaxation can be slow on the femtosecond time scale. A second observation might be that virtually everything fluoresces. According to the Heisenberg uncertainty principle, every transition having a bandwidth of a few hundred nanometers must be associated with a state having a lifetime at least as long as femtoseconds. Where there is absorption, there must be emission, albeit very small in some cases. Ultrafast spectroscopy could make fluorescence more universally applicable. Raman emission is also interesting on the ultrafast time scale. Below 1 ps, resonance Raman emission emerges (12). In part d of Figure 1, on the time scale less than 100 fs, an intense and rapid decay resulting from resonance Raman emission appears. By selecting a narrow emission bandwidth, this peak could be larger than the fluorescence emission. The difference between resonance Raman and fluorescence is understood (13). On this time scale, the electronic states of the absorber are still in phase with the excitation, and the resulting temporally coherent emission process is referred to as resonance Raman. Collisions perturb the energies and, therefore, the phases of these states. The emission process after dephasing is referred to as fluorescence. On the 100-fs time scale, emission is also observed coincident with the laser pulse, as shown in part d. This is Raman emission without resonance enhancement, contributed by the solvent. The value of such technology is evident for Raman spectroscopy. Raman spectra contain structural information, yet their use is limited because of low sensitivity and fluorescence interference. Our hypothetical experiment shows that all of the photons from the resonance Raman process are emitted before 1 ps, where little fluorescence interferes. One could use resonance enhancement with minimal interference
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INSTRUMENTATION from fluorescence. In fact, various techniques have been used to reject flu orescence from Raman emission based on temporal discrimination: time-do main methods with conventional photomultipliers (14) and microchannelplate photomultipliers (15) and fre quency-domain phase nulling with photomultiplier detection (16-18). These methods cannot push Raman spectroscopy to its fullest potential be cause they are a few orders of magni tude too slow. None has achieved rejec tion of fluorescence by more than a fac tor of about 200, and this is not good enough for wide applicability. Thus workers avoid fluorescence in Raman spectroscopy by excitation with nearIR (19), red (20), and UV (21) light. One application of an ultrafast emis sion experiment is very close to achiev ing routine use in analytical chemistry: the measurement of fast fluorescence anisotropy decays. To be widely appli cable, a time resolution of 10 ps is need ed because the reorientation times of moderately small molecules in solvents of low viscosity can be as short as 10 ps. Although a commercial instrument with this time resolution does not exist, photomultiplier technology can now be pushed to this level of performance, particularly with frequency-domain spectroscopy. As with fluorescence life times, early efforts in molecular reori entation measurements also involved the study of laser dyes. Short optical pulses can now be generated through out the visible and UV, allowing appli cability to virtually any fluorescent species of interest. According to Debye (22), the reorien tation time, τΟΓ, of a solute is propor tional to its molecular size. As a param eter for identification, r or is potentially more valuable than either the fluores cence or the phosphorescence lifetime because it contains information per taining more directly to solute struc ture. Anisotropy decays are multiple exponential (23), and their analysis also reveals the hydrodynamic shape of the solute as well as its size. Anisotropy measurements contain a wealth of information. In addition to the structural parameters of size and shape, the symmetry of the excited state relative to the ground state is de termined. Furthermore, the excitedstate symmetry is wavelength depen dent (24). Rather than just using the spectrum for identification, research ers will also routinely use the polariza tion dependence of the spectrum. The value of such a measurement has been demonstrated for a mixture analysis of anthracene derivatives (25). The evolution of the fluorescence spectrum, vibrational relaxation, the
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decay of Raman and resonance Raman emission, and the reorientation of mol ecules occur on ultrafast time scales. The amount of information that can be obtained by time resolution far exceeds the information in steady-state Raman and electronic spectra. These examples refer to an emission experiment, but many other types of experimental ar rangements are used in ultrafast spec troscopy. Beyond photomultipliers: techniques of ultrafast spectroscopy Although present ultrafast lasers can have pulse widths as short as 6 fs (26), 50 fs is more typical. What is lacking for the hypothetical femtosecond emis sion equipment to be a reality is an ultrafast photomultiplier. There are two general approaches to improving the time resolution of the experiment: make the detector faster or gate the light emitted from the sample. Streak cameras. Photomultipliers are limited in speed by an inability to contain a collection of 106 electrons per pulse in closer spatial proximity. A streak camera, which is a distant rela tive of the photomultiplier, operates as fast as 0.6 ps at high repetition rates (27). What a streak camera has in com mon with a photomultiplier is a photocathode. When light strikes the photo-
Figure 2. Schematic of a pump/probe experiment. The pump and probe lasers are modulated at 8 and 13 MHz, with the pump laser chopped at 20 Hz. The arrival time of the probe laser is con trolled by a variable delay line, and its intensity is detected with a photodiode. The signal at 21 MHz is isolated by a lock-in amplifier and is distin guished from any background contribution at 21 MHz by the low-frequency lock-in amplifier.
_ 1000(2.303)2 tifzbcPiP, RKNA R is the repetition rate of the laser; Κ is the photon energy; Ν is Avogadro's number; A is the beam area; ίι and t
