Femtosecond pump beam

An alternative ultrafast TRIR method, originally developed by Hochstrasser and co-workers [26], avoids the requirement of short IR pulses. Here a single frequency of a continuous wave (cw) IR probe source is overlapped with a femtosecond pump beam (again typically in the visible). Time-resolved detection is accomplished by frequency mixing (in a nonlinear crystal located after the sample) of the cw IR probe beam with a second femtosecond visible pulse that has been time delayed relative to the pump beam. This produces a visible pulse at a frequency equal to the sum of the frequency of the original visible pulse plus the IR probe frequency with an intensity related to the IR absorbance of the photo-excited sample. 

Following a two photon excitation of hydrated hydroxyl ions (FhO/NaOH = 55) with femtosecond UV pulses (/.purnp = 310 nm, Eexdtation = 2x4 eV), short-time electron transfer trajectories have been investigated by near-IR and UV absorption spectroscopies at room temperature. The energy of the pump beam is 1011 W cm 2. 

Experiments were performed using a commercial kilohertz femtosecond Ti Sapphire laser system (Spectra Physics) delivering laser pulses at 790 nm with a duration of 110 fs and an energy of 750 pj. The pump beam at 263 nm (third harmonic) was produced by frequency doubling and sum-frequency mixing in two BBO crystals. Then, the pump beam was focused on a 300 pm thick ethylene glycol jet in order to produce electrons by photoionisation of the... 

Next, we consider in detail the results for the A state. A series of femtosecond pump-probe experiments were performed at wavelengths corresponding to the Rydberg states A (t = 0,1,2) of an ammonia monomer (Herzberg 1960 Ziegler 1985). (Note throughout the text that the vibrational levels denote those of unclustered ammonia molecules.) The wavelengths used to access these vibrational levels (in the monomer) were 214 nm, 211 nm, and 208 nm for the pump laser, and 321 nm, 316.5 nm, and 312 nm for the probe laser. For each experiment, the probe beam was appropriately delayed, as discussed earlier. 

The pump beam intensity (Verdi, Coherent Inc.) is controlled by an EOM (LM 0202, Gsanger). With 7 W of pump power we achieve above 650 mW average power from the femtosecond laser. 

Among the best well-known examples of photostability after UV radiation, the ultrafast nonradiative decay observed in DNA/RNA nucleobases, has attracted most of the attention both from experimental and theoretical viewpoints [30], Since the quenched DNA fluorescence in nucleobase monomers at the room temperature was first reported [31] new advances have improved our knowledge on the dynamics of photoexcited DNA. Femtosecond pump-probe experiments in molecular beams have detected multi-exponential decay channels in the femtosecond (fs) and picosecond (ps) timescales for the isolated nucleobases [30, 32-34], The lack of strong solvent effects and similar ultrafast decays obtained for nucleosides and nucleotides suggest that ultrashort lifetimes of nucleobases are intrinsic molecular properties, intimately... 

The basis of the experimental femtosecond CARS apparatus developed by Okamoto and Yoshihara (1990) which is reproduced in Fig. 3.6-10 is essentially the same as that of Leonhardt et al. (1987) and Zinth et al. (1988) with the addition of the possibility to change the polarization of the laser radiation. The main parts of the system are two dye lasers with short pulses and high repetition rates, pumped by a cw mode-locked Nd YAG laser (1064 nm, repetition rate 81 MHz). The beam of the first dye-laser which produces light pulses with 75-100 fsec duration is divided into two parts of equal intensities and used as the pump and the probe beam. After fixed (for the pump beam) and variable (for the probe beam) optical delay lines, the radiation is focused onto the sample together with the Stokes radiation produced by the second laser (DL2), which is a standard synchronously pumped dye laser. The anti-Stokes signal generated in the sample is separated from the three input laser beams by an aperture, an interference filter, and a monochromator, and detected by a photomultiplier. For further details we refer to Okamoto and Yoshihara (1990). 

This technique utilizes a pulse pump-probe experiment and monitors the absorption of a weak probe beam in the presence of a strong pump beam. Fig. 8 depicts the experimental set-up for a two-beam pump-probe experiment, which includes homodyne and heterodyne Kerr gate measurements and polarization-controlled transient absorption measurement. Generally, the input beam is produced from an amplified pulse laser system with 1 KHz repetition rate, which can produce picosecond or femtosecond pulses. This pumping light beam is divided into two beams by a beam-splitter with an intensity ratio of 30 1 therefore, the one with the stronger intensity will act as the pump and the weaker one will be the probe. The position of the sample is where these two beams focus and overlap spatially. The time delay between the pulses from these two beams is controlled by a retroreflec-... 

Fig. 8. Femtosecond two-beam pump-probe experimental set-up for Kerr gate and transient absorption measurement. P polarizers M mirrors PBS Pellicle beam splitter Ap aperture RR retroreflector... 

With femtosecond pump-and-probe experiments fast motion pictures of a vibrating molecule may be obtained, and the time behavior of the wave packets of coherently excited and superimposed molecular vibrations can be mapped. This is illustrated by the following examples dealing with the dynamics of molecular multiphoton ionization and fragmentation of Na2, and its dependence on the phase of the vibrational wave packet in the intermediate state [821]. There are two pathways for photoionization of cold Na2 molecules in a supersonic beam (Fig. 6.100) ... 

Up to now, the most common laser sources used for CARS microscopy are based on Tirsapphire or Nd YVO lasers with pulse durations from tens of femtoseconds up to 10 ps. Different approaches are possible in order to generate pump and Stokes beams use of two femtosecond laser sources electronically synchronized [19], pumping of an optical parametric amplifier (OPA) to produce the Stokes beam and use of the residual pump as pump beam [18], pumping of an optical parametric oscillator (OPO) to obtain the pump beam and use the residual pump light as Stokes [20], using signal and idler beams from a synchronously pumped OPO to provide directly the two excitation beams [11, 21]. 

In a typical time-resolved SHG (SFG) experiment using femtosecond to picosecond laser systems, two (tlnee) input laser beams are necessary. The pulse from one of the lasers, usually called the pump laser, induces the... 

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