Femtosecond measurements

B. J. Schwartz, J. C. King, J. Z. Zhang, and C. B. Harris, Direct femtosecond measurements of single collision dominated geminate recombination times of small molecules in liquids. Chem. Phys. Lett. 203(5-6), 503-508 (1993). 

Figure 3.45. Experimental and theoretical TP induced fluorescence excitation spectra (without (e) from the original Ref. [474]) for (a) compound 58b, (b) compound 78a, (c) compound 58c, (d) compound 78b, and (f) compound 78c. Experimental results (a) femtosecond measurements using fluorescein as standard ( ) nanosecond measurements using fluorescein as standard (o) nanosecond measurements using bis(methylstyryl)ben-zene as standard. Theoretical results solid line. (From Ref. [474] with permission of the American Chemical Society.)...

The lack of well-resolved kinetic data in support of the formation of [P -Ba -BOa] presented a dilemma, since the monomeric BChl is in van der Waals contact with the primary donor, whereas BO is further removed at a distance of 17 A. Rudolf Marcus examined the available kinetic data and reviewed two alternative mechanisms proposed for the reduction of the intermediate electron carrier, BChl. One was the super-exchange mechanism of electron transfer that implicates the existence of a virtually populated [P BA ]-state in the mediation of electron transfer between P and BOa and the other a two-step electron transfer to BO that can be kinetically resolved by an intermediary [P BA"]-state. In view of the lack of resolution of such a state in the data obtained by Martin and Breton , Marcus estimated that the putative [P Ba -BOa] state would have an upper lifetime limit of -1 ps. Of course, undertaking measurements of such a brief kinetic event in the neighborhood of a 3-ps event would demand substantially improved measuring techniques and procedures. With this in mind, Holzapfel et al. extended their femtosecond measurements to look for the intermediary [P -BA"]-state. Their measurements entailed the use of short excitation pulses (60/v), appropriate wavelengths for selective excitation of the primary electron donor P, high time resolution (-100 fs), and sufficiently low excitation intensity to avoid double photon excitation and consequently nonlinear processes. As the results summarized below show, these measurements provided new evidence for the existence of a kinetically resolvable, though extremely transient, intermediary [P -Ba -BCJa] state. 

The main question concerning the primary step in photosynthesis is as follows how the excited state of the primary electron donor P in reaction centers (Res) is converted into a charge transfer state (see / / for review) Is the latter state formed within P directly from its excited state P or does a neutral P donate an electron to neighboring bacteriochlorophyl1 (BL) and bacteriopheophytin (HL) monomers located in L protein subunit (revealed by x-ray studies /2/) Femtosecond measurements yielded contradictory conclusions /3, / The first measurements /5"7/ of holes burned at absorption band of P have not shown any narrow bands for P+Q.A"formation, yet narrow-band features have been observed for different reactions in Res /8,9/. Here we show that in open R.vi ridi s Res the holes burned in the region of 0-0 transition of P are broad (>50 cm ) while in closed Res a narrow hole is observed with weak electron-phonon coupling /10,11/ in agreement with the Stokes shift between absorption and fluorescence spectra at 1.7 K. 

Chemiluminescence. In this method, radiation emitted by excited products is spectroscopically analyzed as it is emitted. The intensities of radiation due to various transitions can be used to determine the population distribution for product states. Modem techniques also allow time-resolved spectra to be observed (intensity as a function of time as well as of wavelength). Measurements in the picosecond region are becoming common and femtosecond measurements are being carried out. 

Femtosecond measurement of nonlinear absorption and refraction in CdS, ZnSe, and ZnS. AppL Phys. Lett, 65, 1739. 

Femtosecond lasers represent the state-of-the-art in laser teclmology. These lasers can have pulse widths of the order of 100 fm s. This is the same time scale as many processes that occur on surfaces, such as desorption or diffusion. Thus, femtosecond lasers can be used to directly measure surface dynamics tlirough teclmiques such as two-photon photoemission [85]. Femtochemistry occurs when the laser imparts energy over an extremely short time period so as to directly induce a surface chemical reaction [86]. 

Kobayashi T 1994 Measurement of femtosecond dynamics of nonlinear optical responses Modern Noniinear Optics part 3, ed M Evans and S Kielich Adv. Chem. Rhys. 85 55-104... 

Venables D S and Schmuttenmaer C A 1998 Far-infrared spectra and associated dynamics in acetonitrile-water mixtures measured with femtosecond THz pulse spectroscopy J. Ohem. Rhys. 108 4935-44... 

Because of the generality of the symmetry principle that underlies the nonlinear optical spectroscopy of surfaces and interfaces, the approach has found application to a remarkably wide range of material systems. These include not only the conventional case of solid surfaces in ultrahigh vacuum, but also gas/solid, liquid/solid, gas/liquid and liquid/liquid interfaces. The infonnation attainable from the measurements ranges from adsorbate coverage and orientation to interface vibrational and electronic spectroscopy to surface dynamics on the femtosecond time scale. 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Hamm P, Urn M and Hochstrasser R M 1998 Structure of the amide I band of peptides measured by femtosecond nonlinear-infrared spectroscopy J. Phys. Chem. B 102 6123-38... 

Kane D J, Taylor A J, Trebino R and DeLong K W 1994 Single-shot measurement of the intensity and phase of a femtosecond UV laser pulse with frequency-resolved optical gating Opt. Lett. 19 1061-3... 

Bardeen C J and Shank C V 1994 Ultrafast dynamics of the solvent-solute Interaction measured by femtosecond four-wave mixing LD690 In n-alcohols Chem. Phys. Lett. 226 310-16... 

Yamaguchl S and Hamaguchl H 1995 Convenient method of measuring the chirp structure of femtosecond white-light continuum pulses App/. Spectrosc. 49 1513-15... 

Bigot J-Y, Portella M T, Schoenlein R W, Bardeen C J, Migus A and Shank C V 1991 Non-Markovian dephasing of molecules in solution measured with three-pulse femtosecond photon echoes Phys. Rev. Lett. 66 1138 1... 

The availability of lasers having pulse durations in the picosecond or femtosecond range offers many possibiUties for investigation of chemical kinetics. Spectroscopy can be performed on an extremely short time scale, and transient events can be monitored. For example, the growth and decay of intermediate products in a fast chemical reaction can be followed (see Kinetic measurements). 

Figure 10-2. Experimental setup for pump and probe measurements. Two femtosecond pulses are focused onto the same spot of the sample. The pump pulse-induced changes A7/T0 of the normalized transmission of the probe pulse are measured as a function of the time delay between the two pulses.

Figure 10-7. (a) Absorption spectrum of 3 LPPP. The arrow indicates the spectral po-.oj, silion of the excitation pulse in the time-re- i solved measurements, (b) PL spectrum for LPPP for low excitation pulse energies, (c) Differential transmission spectrum observed in LPPP after photoexcitation with a femtosecond pulse having a pulse energy of 80 uJ at a wavelength of 400 nm. The arrow indicates the spectral position of the probe pulses used for a more detailed investigation of the gain dynamics. 

Luminescence lifetime spectroscopy. In addition to the nanosecond lifetime measurements that are now rather routine, lifetime measurements on a femtosecond time scale are being attained with the intensity correlation method (124), which is an indirect technique for investigating the dynamics of excited states in the time frame of the laser pulse itself. The sample is excited with two laser pulse trains of equal amplitude and frequencies nl and n2 and the time-integrated luminescence at the difference frequency (nl - n2 ) is measured as a function of the relative pulse delay. Hochstrasser (125) has measured inertial motions of rotating molecules in condensed phases on time scales shorter than the collision time, allowing insight into relaxation processes following molecular collisions. 

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