Laser spectroscopy short pulse generation

Recognizing the faet the seattering process occurs on a very short timescale of 10 s, it becomes evident that short-pulse-width lasers can be used to acquire RR spectra of transient species that can be generated by a photolysis pulse. In favorable cases the species of interest can be prepared and interrogated within the same laser pulse, whereas in other situations a short probe pulse is temporally delayed relative to an initial pump pulse, the latter protocol properly being called time-resolved resonance Raman or TR spectroscopy. 

In this chapter we will discuss the general principles of lasers. Since we mainly consider spectroscopic aspects in this book, we will focus on tunable lasers for laser spectroscopy in the frequency (wavelength) domain and short-pulse lasers for spectroscopy in the time domain. Short-pulse lasers are also required for the generation of ultra-intense laser pulses, the use of which has opened up a new field of spectroscopy ultra-intense laser/matter interaction. In addition to the many types of spectroscopically interesting lasers, we will also cover a number of the fixed-frequency lasers that are used to pump them. For more detailed accounts of the field of laser physics, frequently also referred to as quantum electronics, we refer the reader to standard textboolcs [8.1—8.13]. 

While the previous chapter emphasized the high speotral resolution achievable with different sub-Doppler techniques, this chapter concentrates on some methods which allow high time resolution. The generation of extremely short and intense laser pulses has opened the way for the study of fast transient phenomena, such as molecular relaxation processes in gases or liquids due to spontaneous or collision-induced transitions. A new field of laser spectroscopy is the time-resolved detection of coherence and interference effects such as quantum beats or coherent transients monitored with pulse Fowoiev transform spectroscopy. 

To carry out a spectroscopy, that is the structural and dynamical determination, of elementary processes in real time at a molecular level necessitates the application of laser pulses with durations of tens, or at most hundreds, of femtoseconds to resolve in time the molecular motions. Sub-100 fs laser pulses were realised for the first time from a colliding-pulse mode-locked dye laser in the early 1980s at AT T Bell Laboratories by Shank and coworkers by 1987 these researchers had succeeded in producing record-breaking pulses as short as 6fs by optical pulse compression of the output of mode-locked dye laser. In the decade since 1987 there has only been a slight improvement in the minimum possible pulse width, but there have been truly major developments in the ease of generating and characterising ultrashort laser pulses. 

With development of ultrashort pulsed lasers, coherently generated lattice dynamics was found, first as the periodic modulation in the transient grating signal from perylene in 1985 by De Silvestri and coworkers [1], Shortly later, similar modulation was observed in the reflectivity of Bi and Sb [2] and of GaAs [3], as well as in the transmissivity of YBCO [4] by different groups. Since then, the coherent optical phonon spectroscopy has been a simple and powerful tool to probe femtosecond lattice dynamics in a wide range of solid... 

Recent technical developments in laser Raman spectroscopy have made it possible to measure the Raman spectra of short-lived transient species, such as electronically excited molecules, radicals and exciplexes, which have lifetimes on the order of nano- (10-9) and pico- (10-12) seconds. These shortlived species may be generated by electron pulse radiolysis, photo-excitation and rapid mixing. However, the application of electron pulse radiolysis is limited in its adaptability and selectivity, while rapid mixing is limited by mixing rates, normally to a resolution on the order of milliseconds. Thus, photoexcitation is most widely used. 

Photolysis is a selective way to make radicals from a suitable precursor. It is very clean and particularly useful in emission experiments. The energy required to photolyze a precursor is typically about 6 electron volts. This means that photolysis sources will be pulsed lasers or resonance lamps. Lasers such as XeCl (A. = 308 nm) or ArF (A = 193 nm) with pulse widths of about 10 ns and repetition rates of around 100 Hz will thus make transients species in short bursts. It is a difficult task to couple such a system to a Fourier transform spectrometer but it has been done in a dynamics study [23]. Resonance lamps, while providing the appropriate energy, usually do not provide the flux necessary to generate the high concentrations of unstable species needed for a successful spectroscopy experiment [24],... 

Laser flash photolysis is one of the most efficient methods for the direct spectroscopic observation of free radicals and for monitoring the kinetics of formation and decay in real-time. This method is an extension of conventional flash photolysis method [26] that was invented by Norrish and Porter in 1949, and who were awarded by the Nobel Prize in 1967. We have used this approach to investigate the generation and reactions of free radicals with DNA. In this technique, a laser light pulse is used to produce short-lived intermediates in solution contained in an optical cuvette, and the kinetics of their formation and decay are monitored by transient absorption spectroscopy. The apparatus we used is shown in Figure 4.1. 

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