Titanium synchronized

The diffusion of an H atom over the very well dried, but not dehydroxylated, support surface can be described as hopping from one site to another, or alternatively as a synchronous hopping of a proton and an electron (25., 28). Each hopping consists of a simultaneous reduction of the receiving titanium cation and an oxidation of the leaving site, which may explain the similarity in activation energy for H2 chemisorption on and desorption from the support. 

Here, the principal features and characteristics of the ultrafast laser systems used are briefly summarized. Besides the titanium sapphire laser which acts as the workhorse in nearly all of the discussed experiments, a synchronously pumped dye laser is employed to study the ultrafast dynamics of Nas on a picosecond timescale (see Sect. 3.2.2). For measurements with femtosecond time resolution and wavelengths located between 600 and 625 nm a synchronously titanium sapphire pumped optical parametric oscillator followed by frequency doubling is used. To investigate the Nas C state, two mode-locked titanium sapphire lasers have been synchronized. In all cases the essential parameter of the generated laser pulses, the pulse width, has to be determined. This problem is solved by an autocorrelation technique. Hence, the principles of an autocorrelator are briefly described at the end of this section. 

Synchronization of Two Mode-Locked Titanium Sapphire Lasers. Besides utilizing OPOs - described above - the use of two independently tunable ultrafast laser sources is possible. This is of special interest while using picosecond laser sources, since the efficiency of nonlinear optical processes is, due to the peak power, much lower than for equivalent femtosecond processes. To achieve the conditions for the pumpfeprobe technique the pulse trains of the two independent lasers have to be synchronized. A successful approach to this problem is described below. For further details on the design of the appropriate stabilization see [50]. 

Two mode-locked titanium sapphire lasers (Spectra Physics, Tsunami) with.l.4ps pulse duration and a pulse repetition rate of 80 MHz are synchronized using the master-slave technique [222]. The actual repetition rate of one of the two lasers, the so-called master, serves as a reference oscillator. The second laser, the so-called slave, is matched to the repetition rate of the master. This means that the slave oscillator follows the changes of the master s resonator length. A servo loop is used to realize the alignment procedure. 

Fig. 2.15. (a) Cross correlation trace of the synchronized titanium sapphire lasers (averaging time 1 s) obtained by SHG in an LBO crystal with cross correlation width AtcToss = 2.4 ps. The pulse widths of the lasers are both 1.4ps. (b) Phase noise spectral density Sj (/) for open and closed servo loop. Note the break in the scale of Sj (/). The figures are taken from [223]... 

Today the author would prefer an arrangement with a titanium sapphire laser pumping synchronously an OPO (see Sect. 2.1.1). 

The ultrafast photodissociation dynamics of the Na3 C state was analyzed with time-resolved two-color TPI spectroscopy in the picosecond regime. The two excitation wavelengths required, independently tunable for the pump and the probe pulse, were generated by a home-built synchronization of two mode-locked titanium sapphire lasers. The deconvoluted real-time spectra can be well described by a single exponential decay with a time constant strongly... 

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