Nanosecond pump-probe

In conventional nanosecond pump-probe dispersive TRIR experiments, also described previously, kinetic data are collected at one frequency at a time. These data can then be used to construct a series of time-resolved IR spectra. Thus, in the dispersive experiment kinetic data are used to construct spectra, and in the step-scan experiment spectral data are used to derive kinetics. 

Rotational recurrences may be detected in polarization selected spontaneous fluorescence (provided the photodetector has a sufficiently fast response) or by a variety of sub-nanosecond pump/probe schemes (Felker and Zewail, 1987 Felker, 1992 Hartland, et al. 1992 Joireman, et al.. 1992 Smith, et al., 2003a,b). 

In conventional nanosecond pump-probe dispersive TRIR experiments, spectra are measured one frequency at a time. The pump source is typically a nanosecond laser the probe source can be broadband IR light from a globar or tunable IR light from a CO laser or a semiconductor diode laser. 

Fig. 3.36. Excited electronic states A, B, B, C, and D of Nas, revealing a pronounced substructure. While A, B, B, and the energetically lower part of the C state were measured by resonant TPl the D and parts of the C state were detected by depletion spectroscopy. The listed lifetimes were estimated by nanosecond pump probe spectroscopy (taken from [369])...

The sodium trimer excited to the electronic C state can be regarded as a fascinating model system, which manifests ultrafast predissociation dynamics. While stationary and nanosecond-pump probe spectroscopy gave the first hints that this excited state photodissociates rather fast, real-time TPI spectroscopy opens a window to directly observe these ultrafast processes. But let us first start with a short review of the spectroscopy of this excited electronic state. 

For all other bands the lifetimes could not be measured by the nanosecond pump probe technique but were estimated to be less then 5 ns. This suggested that predissociation occurs for at least the highly excited vibrational levels. 

Abstract Far-ultraviolet (FUV) absorption spectroscopy provides molecular information about valence electronic transitions a, n, and Jt electron excitation and charge transfer (CT). FUV spectral measurements of liquid water and aqueous solutions had been limited, because the absorptivity of liquid water is very intense (absorptivity 10 cm at 150 nm). We have developed an attenuated total reflection (ATR)-type FUV spectrophotometer in order to measure FUV spectra of liquid water and aqueous solutions. The ATR-FUV spectroscopy reveals the features of the valence electronic transition of liquid water. This chapter introduces a brief overview of the first electronic transition (A. Y) of liquid water (Sect. 4.1) and the FUV spectral analyses (140-300 nm) of various aqueous solutions including how the hydrogen bonding interaction of liquid water affects the A <— X transition of water molecules (Sect. 4.1) how the A Y bands of water molecules in Groups 1, 11, xm, and lanthanoid (Ln +) electrolyte solutions are associated with the hydration states of the metal cations (Sects. 4.2 and 4.3) how the protonation states of amino acids in aqueous solutions affect the electronic transition of the amino acids (Sect. 4.4) and the analysis of O3 pulse-photolytic reaction in aqueous solution using a nanosecond pump-probe transient FUV spectrophotometer (Sect. 4.5). 

Unfortunately, femtosecond laser pulses are not so readily predisposed to study collisions between atoms and molecules by the pump-probe approach. The reason is that, typically, the time between collisions in the gas phase is on the order of nanoseconds. So, with laser pulses of sub-lOOfs duration, there is only about one chance in ten thousand of an ultrashort laser pulse interacting with the colliding molecules at the instant when the transfer of atoms is taking place in other words, it is not possible to perform an accurate determination of the zero of time. 

Although very detailed, fundamental information is available from ultrafast TRIR methods, significant expertise in femtosecond/picosecond spectroscopy is required to conduct such experiments. TRIR spectroscopy on the nanosecond or slower timescale is a more straightforward experiment. Here, mainly two alternatives exist step-scan FTIR spectroscopy and conventional pump-probe dispersive TRIR spectroscopy, each with their own strengths and weaknesses. Commercial instruments for each of these approaches are currently available. 

The pump-probe method provides the solution to this nanosecond barrier. Here, two light pulses are generated one to excite the sample (prepare the excited state) and one to probe the system at a given time after excitation (Figure 10.8). 

The outlook is good for applications of these picosecond methods to an increasing number of studies on reactive intermediates because of the limitations imposed by the time resolution of nanosecond methods and the generally greater challenges of the use of a femtosecond spectrometer. The pump-probe technique will be augmented in more widespread applications of the preparation-pump-probe method that permits the photophysics and photochemistry of reactive intermediates to be studied. 

An analysis of the time dependent transient photo-reflectance gives us a good estimate of the effective pair recombination rate [8], The inset in Fig. 1(b) shows the average reflectance change as a function of the delay between pump and probe pulses at 6 K. In accordance with other BCS superconductors the effective recombination time is found to be a few nanoseconds. One important remark is that, within the experimental time resolution, no evidence of multiple decays is found. In fact, ultra-fast pump-probe measurements on MgB2 [12, 13] did not find any evidence for a double relaxation down to the ps regime. 

Nanosecond pump probc time-resolved resonance Raman experiments were carried out with two Nd YAG lasers (At=7 ns) which provide the pump- (532 nm, 4 mJ) and probe pulses (416 nm, 100 mJ), a triple polychromator equipped with an intensified photodiode array, and a slow spinning cell (25tpm). For every sample, spectra were taken for delay times of At=-20, 0, 20,100 and 500 ns, 1,10,100 and 500 is, and 1ms. ... 

A simplified view of the early processes in electron solvation is given in Figure 7. Initially, electron pulse radiolysis was the main tool for the experimental study of the formation and dynamics of electrons in liquids (Chapter 2), first in the nanosecond time range in viscous alcohols [23], later in the picosecond time range [24,25]. Subsequently, laser techniques have achieved better time resolution than pulse radiolysis and femtosecond pump-probe laser experiments have led to observations of the electron solvation on the sub-picosecond to picosecond time scales. The pioneering studies of Migus et al. [26] in water showed that the solvation process is complete in a few hundreds of femtoseconds and hinted at the existence of short-lived precursors of the solvated electron, absorbing in the infrared spectral domain (Fig. 8). The electron solvation process could thus be depicted by sequential stepwise relaxation cascades, each of the successive considered species or... 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

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