Ultrashort infrared pulses

In ultrafast, time-resolved infrared absorption measurements by the pump-probe method, the sample is first excited by an ultrashort pump pulse, and then irradiated by an ultrashort infrared pulse (probe pulse) after a certain delay time from the excitation by the pump pulse. The delay time of the probe pulse from the pump pulse is usually changed by the difference in the optical path lengths of the pump and probe pulses (a delay time of 1 ps arises from a path difference of about 0.3 mm). When the infrared spectrum of a molecule in an excited electronic state is measured, pulses in the ultraviolet to visible region are used for the pump purpose, and pulses in the infrared region are used for the probe purpose. When a vibrationally excited molecule is the target of such a measurement, pulses in the infrared region are used for both the pump and probe purposes. The transient (or time-resolved) infrared absorption spectra by this method are usually measured as the difference in absorption intensities for the probe pulses between the measurements with the pump pulses and those without the pump pulses. 

Time-Resolved Spectroscopic Measurements with Narrow-Band Ultrashort Infrared Pulses and Applications... 

Given the lack of success with the phase-tailored UV pulse we tried a different approach a two-pulse scheme, where a weak-field ultrashort UV pulse is combined with a strong infrared (IR) field [7]. The strong IR field affects the nuclear dynamics such that the propagator AT, in Eq. (1) in this case describes the nuclear dynamics under the influence of Vt + fijEm (r), where fii is the dipole moment in the electronic state i. The idea that this approach may work is based on the fact that the nuclear dynamics no longer takes place on the pure Bom-Oppenheimer potentials and this may speed up the process. Various applications of IR+UV two-pulse schemes have been discussed recently [9-12]. 

THEORY OF LASER CONTROL OF VIBRATIONAL TRANSITIONS AND CHEMICAL REACTIONS BY ULTRASHORT INFRARED LASER PULSES... 

This approach has the potential to resolve the time evolution of reactions at the surface and to capture short-lived reaction intermediates. As illustrated in Figure 3.23, a typical pump-probe approach uses surface- and molecule-specific spectroscopies. An intense femtosecond laser pulse, the pump pulse, starts a reaction of adsorbed molecules at a surface. The resulting changes in the electronic or vibrational properties of the adsorbate-substrate complex are monitored at later times by a second ultrashort probe pulse. This probe beam can exploit a wide range of spectroscopic techniques, including IR spectroscopy, SHG and infrared reflection-adsorption spectroscopy (IRAS). 

NEW TECHNIQUES OF TIME-RESOLVED INFRARED AND RAMAN SPECTROSCOPY USING ULTRASHORT LASER PULSES... 

These simplifications have been made (a) interband transitions are not counted. This assumption imposes a restriction on the maximum frequency of laser pulses, which for the CNT is in the near infrared [11]. (b) the solution for the field component is in the class of rapidly decreasing functions, (c) the relaxation time t is large enough for the common durations of ultrashort laser pulses. 

By irradiating the vapors of metals such as Ba and Cs with ultrashort laser pulses, infrared pulses are generated by the process of stimulated electronic Raman scattering. 

Infrared pulses at frequency Vp, are generated as the difference frequency between two ultrashort pulses at frequencies vj and V2 (i.e., vp, = vj - vj vj > V2>. Examples of nonlinear crystals used for the generation of a difference frequency are LiI03 and AgGaS2. 

In practice, an electromagnetic pulse with an infinitely short width does not exist, but ultrashort laser pulses are now used for various spectroscopic measurements. Terahertz spectrometry described in Chapter 19 is based on femtosecond laser pulses. In Chapter 20, time-resolved infrared spectroscopic methods using picosecond to femtosecond laser pulses are described. Such ultrashort laser pulses have large spectral widths in the frequency domain. Let us discuss the relation between the pulse width in the time domain and its spectral width expressed in either frequency or wavenumber. 

The most direct and easy way consists in focusing the laser pulse onto a solid target and to collect the radiation emitted by the produced plasma. The wide emitted spectrum extends from infrared to X-rays and it is produced by different physical mechanisms Bremsstrahlung, recombination, resonant lines, K-shell emission from neutral (or partially ionized) atoms. In particular, this latter mechanism has been recognized, since a decade, as a way of producing ultrashort monochromatic radiation pulses at energy up to several keV. 

Figure 1. Testing the Keldish limit [1, 2] to ionization by intense infrared femtosecond/picosecond laser pulses used for control of chemical reactions [3, 4], (a) Electronic ground state embedded in a typical model potential curve with the ionization potential Es = 12.9 eV. (b) Intense ( o = 35.5 GV/m"1, Iq = 3.3 x 1014 W/cm2), ultra-short (tp = 0.5 ps), infrared (l/X = 3784 cm" ) laser pulse, (c) Expectation value for the position of the election, which is driven by the laser held shown in panel (b) [compare with ro = 122 A, Eq. (3)]. (d) Electron energy. These model calculations demonstrate that even very intense (/ > /Keldish) ultrashort 1R laser pulses may not cause ionization that is, the simple estimates (1)—<4) [1, 2] are not applicable.

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