Light continuum probe beam

Scheme 8.1 Schematic layout of an ultrafast spectroscopy setup. Continuous line, fundamental (800 nm) dashed line white light continuum probe dotted line pump beam M mirror PM parabolic mirror L lens OF optical fiber S sample ...

The femtosecond transient absorption studies were performed with 387 nm laser pulses (1 khz, 150 fs pulse width) from an amplified Ti Sapphire laser system (Model CPA 2101, Clark-MXR Inc). A NOPA optical parametric converter was used to generate ultrashort tunable visible pulses from the pump pulses. The apparatus is referred to as a two-beam setup, where the pump pulse is used as excitation source for transient species and the delay of the probe pulse is exactly controlled by an optical delay rail. As probe (white light continuum), a small fraction of pulses stemming from the CPA laser system was focused by a 50 mm lens into a 2-mm thick sapphire disc. A schematic representation of the setup is given below in Fig. 7.2. 2.0 mm quartz cuvettes were used for all measurements. 

Fig. 7.2 Two-beam experimental setup for femtosecond transient absorption studies using a white light continuum. A commercially available CPA 2101 laser system delivers the pulses. Ultrashort tunable visible pulses are obtained by the NOPA optical parametric converter. A chopper wheel is used to cut every second pump pulse in order to compare the signal with and without the pump. The white light continuum is generated by a sapphire disc. The time delay between the pump and probe pulses is adjusted by the optical delay rail...

Essentially, a small part of the laser pulse train that is ultimately used to trigger the photocathode is split off to create a synchronized laser probe pulse train. The probe line is equipped with different nonlinear optical devices that permit the tunability of the probe beam from the near UV to the NIR. Available probe sources include the laser fundamental (790 nm) and second harmonic (395 nm), a white-light continuum (470-750 nm) generated in a sapphire plate, and a continuously tunable Optical Parametric Amplifier (470-750 nm, 1000-1600 nm, and 240-375 nm by SHG), able to deliver light pulses shorter than 30 fs after compression. 

Significantly more information is available when the probe pulse is a white-light continuum and a spectrograph is used to disperse the spectral distribution of the beam transmitted by the sample. Under these conditions, one obtains an absorption spectrum of transient species formed, or ground-state species removed by the ini-... 

The basic scheme of Fig. 3 also applies to the continuum measurements. Either the pump or the probe beam is delayed. For the same reason as explained above, it is better to delay the pump beam. In addition, the light of the pump and the probe beams should be polarized with an angle between the polarization directions of 54.7°. 

Figure 11.3 Schematic representation of the experimental setup. The following notations have been used BS, beam splitter of the fundamental 775 nm beam shutter 1 and shutter 2, two motorized shutters that block the probe and pump beams, respectively Ml, M2, M3,and M4, high reflection dielectric mirrors at 775 nm, 45° X,/2, aXjl plate Cap2, calcium fluoride plate, used for white light continuum generation M5,...

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