Fourier transform laser pulse measurement

To introduce the application of ultrashort laser sources in microscopy, we want to review some properties of femtosecond pulses first for a comprehensive introduction the reader may refer to one of the established textbooks on femtosecond lasers (Diels and Rudolph 2006). The most important notion is the Fourier transform relation between the temporal shape of a pulse and the spectrum necessary to create it. This leads to the well-known time-bandwidth product for the pulse temporal width (measured as full width at half maximum, FWHM) At and the pulse spectral width Av. 

The laser heating technique can be applied to perform temperature jumps by irradiating short laser pulses at the sample container. Ernst et al. (54) used such a temperature jump protocol to perform stop-and-go experiments. After the start of the laser pulse, the temperature inside the sample volume is raised to the reaction temperature, the conversion of the adsorbed reactants proceeds, and the H MAS NMR measurement is performed. After the laser pulse is stopped, the temperature inside the sample volume decreases to ambient temperature, and the C MAS NMR measurement is made. Subsequently, the next laser pulse is started and, in this way, the reaction is recorded as a function of the reaction time. By use of the free-induction decay and the reaction time as time domains and respectively, a two-dimensional Fourier transformation leads to a two-dimensional spectrum, which contains the NMR spectrum in the Ej-dimension and the reaction rate information in the Ts-dimension (54,55). 

The experimental configuration of the pump-probe experiment is similar to Ref. [5]. A home built non-collinear optical parametric amplifier (nc-OPA) was used as a pump, providing Fourier-transform-limited 30 fs pulses, which could be spectrally tuned between 480-560 nm. In all experiments white-light generated in a sapphire crystal using part of the fundamental laser (800 nm), was used as probe light. In the pump-probe experiments the pump was tuned to the S2 0-0 band for carotenoids with n>l 1. In the case of M9, it was not possible to tune the nc-OPA to its 0-0 transition, and hence another nc-OPA tuned to 900 nm was frequency doubled and used for excitation. In addition to conventional transient absorption pump-probe measurements, we introduce pump-deplete-probe spectroscopy, which is sensitive to the function of an absorbing state within the deactivation network. In this technique, we... 

Figure 5.4, one can easily understand why the interfacial electron transfer should take place in the 10-100 fsec range because this ET process should be faster than the photo-luminescence of the dye molecules and energy transfer between the molecules. Recently Zimmermann et al. [58] have employed the 20 fsec laser pulses to study the ET dynamics in the DTB-Pe/TiC>2 system and for comparison, they have also studied the excited-state dynamics of free perylene in toluene solution. Limited by the 20 fsec pulse-duration, from the uncertainty principle, they can only observe the vibrational coherences (i.e., vibrational wave packets) of low-frequency modes (see Figure 5.5). Six significant modes, 275, 360, 420, 460, 500 and 625 cm-1, have been resolved from the Fourier transform spectra of ultrashort pulse measurements. The Fourier transform spectrum has also been compared with the Raman spectrum. A good agreement can be seen (Figure 5.5). For detail of the analysis of the quantum beat, refer to Figures 5.5-5.7 of Zimmermann et al. s paper [58], These modes should play an important role not only in ET dynamics or excited-state dynamics, but also in absorption spectra. Therefore, the steady state absorption spectra of DTB-Pe, both in...

To analyze the OKE transient, it is necessary to measure the lAF of the incident laser pulse. To do this, we put a KDP crystal in place of the sample and measured the intensity of noncollinear second harmonic signal. On calculating the Fourier transform of the signal, special care should be taken to determine accurately the time origin for the OKE measurement. For this purpose, we have measured the lAF very carefully and have paid a particular attention to the mechanical stability of the optical system between the OKE and lAF measurements. 

This property has been used to measure the coherence properties of laser pulses [28]. Because it is readily obtainable by Fourier transform from the... 

It is important to have reliable laser diagnostics, preferably on a shot-to-shot basis this is possible for a 10 Hz system. To characterise the temporal profile of the pulses, single-shot spectra and autocorrelation data can establish whether the laser pulses are Fourier-transform limited. For lasers with a sech profile, a product AvAtxO.32 should be achieved. It is also important to monitor the focal spot, its Airy disc and average intensity. Alternatively, a reasonable measure of the focused intensity can be obtained using Xe gas and the known threshold intensities for producing the various stages of ionization [12]. 

The dephasing time, T2, can be measured by the photon echo technique or determined from the homogeneous width of saturation spectroscopy, which is a Fourier transform of the former, as easily seen from Eq. (5.35). When a sample is irradiated with three consecutive laser pulses at times, 0, t2, and f3, an echo pulse is emitted at time, t2 + t. This is called stimulated photon echo. Several additional echo techniques have been proposed. 

Neutral homoleptic platinum carbonyls remain unknown under normal laboratory conditions. Platinum monocarbonyl, PtCO, has been prepared by laser ablation of Pt in the presence of GO, and its pure rotational spectrum measured between 6,500 and 20,000 M Hz, using a cavity-pulsed jet Fourier transform microwave spectrometer. A Pt-G bond length of 1.760 and a G-O bond length of 1.148 A were measured, and may be rationalized in the conventional terms of cr-donation from and vr-backbonding to the GO. ... 

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