CPM laser

The shortest optical pulses actually used so far (1998) in ultrafast spectroscopic experunents were obtained by Shank and co-workers from an amplified CPM laser [ ]. In these extraordinary experiments, a sequence of a pair of prisms... 

The earliest subpicosecond systems incorporated dye laser technology. Shank, Ippen, and their colleagues at the Bell Laboratories [34] were the first to develop mode-locked subpicosecond lasers and to show how to compress pulses to very short values. With the colliding-pulse mode-locked (CPM) laser they achieved reduced pulse widths well into a subpicosecond range. Two approaches based on synchronously pumped dye lasers and colliding pulse dye lasers are commonly employed to produce subpicosecond pulses. These are briefly discussed below. 

Until recently, the pulses used in those experiments were the shortest optical pulses characterized. The transition-state spectroscopy of Zewail and Bernstein [16,17,18,19, 20,21 and 22] exploited an amplified CPM laser after frequency doubling and/or continuum generation. The chemical systems that were most easily studied, however, were those that could be stimulated either by the 620 nm output of the CPM directly or after frequency doubling to 310 nm. In addition, the CPM laser and its contemporary, more tunable alternative, the pulse-compressed, synchronously pumped dye laser [H], were tools that could be effectively used only by researchers with extensive backgrounds in lasers and optics. 

Experimental Setup. In general, at the heart of a continuum pump-probe experiment is a high repetition rate laser. This can be a passively mode-locked CPM laser which is pumped by a cw Ar laser and which produces pulses typically shorter than 100 fsec around 620 nm with a pulse energy of approximately 0.1 nJ. Alternatively, a mode-locked Nd-YAG laser (532 nm) pumps a dye laser. In that case pulses at somewhat shorter wavelengths can be obtained, which are higher in energy when the same repetition rate is used (typically 0.5 nJ), but the pulses are usually longer, even after pulse compression (>100 fsec). 

A completely different method was developed and applied to the study of ultrafast processes in bacteriorhodopsin. Pulses from a CPM laser were amplified, and then the light was chirped in an optical fiber. With the use of pulse compression ultrashort pulses of 6-12 fsec were obtained. The ultrashort pulses correspond to a broad spectral range because of the Heisenberg uncertainty principle. We refer the reader elsewhere for further details. 

At a proper choice of the amplifying gain and the absorption losses, this situation will automatically be realized in the passively mode-locked ring dye laser. It leads to an energetically favorable stable operation, which is called colliding-pulse mode (CPM) locking, and the whole system is termed a CPM laser. This mode of operation results in particularly short pulses down to 50 fs. There are several reasons for this pulse shortening ... 

As one important example, the introduction of the prism-controlled, colliding-pulse, mode-locked (CPM) dye laser [12,13] led almost innnediately to developments in measurement teclmique with pulses of less than 100... 

Figure B2.1.7 Transient hole-burned speetra obtained at room temperature with a tetrapyrrole-eontaining light-harvesting protein subunit, the a subunit of C-phyeoeyanin. Top fluoreseenee and absorption speetra of the sample superimposed with die speetnuu of the 80 fs pump pulses used in the experiment, whieh were obtained from an amplified CPM dye laser operating at 620 mn. Bottom absorption-diflferenee speetra obtained at a series of probe time delays.

Several laser systems have been used in our time-resolved PM measurements. For the ultrafast measurements, a colliding pulse mode-locked (CPM) dye laser was employed [11]. Its characteristic pulsewidth is about 70 fs, however, its wavelength is fixed at 625 nin (or 2.0 cV). For ps measurements at various wavelengths two synchronously pumped dye lasers were used (12], Although their time resolution was not belter than 5 ps, they allowed us to probe in the probe photon energy range from 1.25 cV to 2.2 cV. In addition, a color center laser... 

The advent of ultrafast pump-probe laser techniques62 and their marriage with the TOF method also enables study of internal ion-molecule reactions in clus-ters.21,63-69 The apparatus used in our experiments is a reflectron TOF mass spectrometer coupled with a femtosecond laser system. An overview of the laser system is shown in Figure 4. Femtosecond laser pulses are generated by a colliding pulse mode-locked (CPM) ring dye laser. The cavity consists of a gain jet, a... 

Free phase optimization ofCpMn(CO)% as a fragment of CpM nfCO)j by means of shaped femtosecond laser pulses... 

Some of the earlier experiments were carried out using our home-built colliding pulse modelocked (CPM) ring dye laser. Equipped with two excimer... 

Where R is the reflectivity and d is the thickness. Very accurate values of R and T are needed when the absorptance, (id, is small. The technique of photothermal deflection spectroscopy (PDS) overcomes this problem by measuring the heat absorbed in the film, which is proportional to ad when ad 1. A laser beam passing just above the surface is deflected by the thermal change in refractive index of a liquid in which the sample is immersed. Another sensitive measurement of ad is from the speetral dependence of the photoconductivity. The constant photocurrent method (CPM) uses a background illumination to ensure that the recombination lifetime does not depend on the photon energy and intensity of the illumination. Both techniques are capable of measuring ad down to values of about 10 and provide a very sensitive measure of the absorption coefficient of thin films. 

Laser flash photolysis of [CpM(CO>3]2 (M = W, Mb, and Cr) provides a convenient source of CpM(CO)3, an organometallic free radical with 17 valence electrons. It is a transient and highly reactive species. Depending on the circumstances and the other reagents present, the radical will dimerize, undergo halogen and hydrogen atom abstraction reactions, and electron transfer reactions. With tetramethyl-phenylenediamine, there is a cyclic process of electron transfer steps, the net result of which is the catalyzed disproportionation of the metal radical. 

The chemistry of the CpM(CO)3 radicals occurs on the microsecond time scale, and flash photolysis with optical detection is the technique most generally applicable. The radical is created by the photohomolysis of the stable dimer, [CpM(CO)3l2, available commercially for M = Mo and W. The excitation sources used were Nd-YAG 532 nm) and flashlampi-pumped dye lasers (X.g c 490-525 nm). When shorter wavelengths are used, the desired process in accompanied by increasing amounts of CO loss this is in general undesirable because it introduces other transients into the system. 

Radical recombination. The results p>ertaining to many of the reactions in Scheme I will now be presented, starting with (a). When a solution of the metal dimer is subjected to a laser flash, -25% of the dimer is dissociated in a typical experiment a 20 iM solution of [CpM(CO)3l2 (M = Mo, W) yields about 25 pM of the dimer. A slightly different procedure was used for M = Cr, but with comparable results. The recombination of the radicals occurs over about 300 ps and follows second-order kinetics. A typical experiment for the molybdenum radical and the fit to second-order kinetics is shown in Figure 1. The rate constants (fc/lO L mol s ) in acetonitrile at 23 °C arc Cr 0.27, Mo 2.16, and W 4.7. In other organic solvents the rate constants are comparable to these, reflecting the relatively small differences in viscosity. In aqueous solution (C5H4C02 )Mo(CO)3 has it/10 L mol s = 3.0. 

Radical disproportionation induced by electron transfer. Several interesting and interrelated phenomena occur when N J I, N -tetramethyl-1,4-phenylenediamine (TMPD) is added to the solution of [CpM(CO)3l2 prior to the laser flash. As expected from the electrode potential of TMPD /TMPD, 0.16 V vs SSCE in acetonitrile, the first event is the rapid growth of the intense absorption band of the amine radical cation centered at 613 nm (e = 1.2 X 10 L mol cm" ). This absorption then fades fairly rapidly. The fading was quite unexpected, since TMPD " normally persists indefinitely. This phenomenon is illustrated in Figure 2. Indeed, when independently-prepared TMPD was injected into the cuvette after the laser flash its blue color persisted indefinitely. 

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