Spectroscopy in the Pico-to-Attosecond Range

For measurements of very fast relaxation processes that demand a time resolution below 10 s most detectors (except the streak camera) are not fast enough. Here the pump-and-probe technique is the best choice. It is based on the following principle shown in Fig. 6.96. 

The molecules under investigation are excited by a fast laser pulse on the transition 0 1). A probe pulse with a variable time delay t against the pump pulse probes the time evolution of the population density Niit). The time resolution is only limited by the pulse width AT of the two pulses, but not by the time constants of the detectors  

In early experiments of this kind a fixed-frequency mode-locked Ndiglass or Nd YAG laser was used. Both pulses came from the same laser and fortuitious coincidences of molecular transitions with the laser wavelength were utilized [806]. The time delay of the probe pulse is realized, as shown in Fig. 6.97, by beam splitting and a variable path-length difference. Since the pump and probe pulses coincide with the same transition i) A ), the absorption of the probe pulse measured as a function of the delay time r, in fact, monitors the time evolution of the population difference [Nk(t) — Ni(t)]. A larger variety of molecular transitions becomes accessible if the Nd YAG laser wavelength is Raman shifted (Vol. 1, Sect. 6.8) into spectral regions of interest [807]. 

A broader application range is opened by a system of two independently tunable mode-locked dye lasers, which have to be pumped by the same pump laser in order to synchronize the pump and probe pulses [808]. For studies of vibrational levels in the electronic ground states of molecules the difference frequency generation of these two dye lasers can be used as a tunable infrared source for direct excitation of selected levels on infrared-active transitions. Raman-active vibrations can be excited by spontaneous or stimulated Raman transitions (Chap. 3). Another useful short-pulse source for these experiments is a three-wavelength Ti sapphire laser, where two of the wavelengths can be indepently tuned [811]. 

In addition, short-pulse tunable optical parametric oscillators have been realized, where the pump wavelength and the signal or idler waves can be used for 

---