Pump pulse determinations

Fig. 6.8. A Principle of frequency-multiplexed CARS microspectroscopy A narrow-bandwidth pump pulse determines the inherent spectral resolution, while a broad-bandwidth Stokes pulse allows simultaneous detection over a wide range of Raman shifts. The multiplex CARS spectra shown originate from a 70 mM solution of cholesterol in CCI4 (solid line) and the nonresonant background of coverglass (dashed line) at a Raman shift centered at 2900 cm-1. B Energy level diagram for a multiplex CARS process. C Schematic of the multiplex CARS microscope (P polarizer HWP/QWP half/quarter-wave plate BC dichroic beam combiner Obj objective lens F filter A analyzer FM flip mirror L lens D detector S sample). D Measured normalized CARS spectrum of the cholesterol solution. E Maximum entropy method (MEM) phase spectrum (solid line) retrieved from (D) and the error background phase (dashed line) determined by a polynomial fit to those spectral regions without vibrational resonances. F Retrieved Raman response (solid line) calculated from the spectra shown in (E), directly reproducing the independently measured spontaneous Raman response (dashed line) of the same cholesterol sample...

The values of /labs were determined from the experimentally measured absorption anisotropy with Eq. 6. In viscous polyTHF, the rotational movement of dye molecules on a lOOps time scale is assumed to be negligible, and thus, does not reduce the limiting value of anisotropy. For the concentrated fluorene solutions (5 x 10 M, 1 mm cuvette), the anisotropy Tabs was not affected by FOrster depolarization mechanisms [13] due to the short time delay, tD, between probe and pump pulses when td/ti 1 [35]. 

To this aim we study the DHA/VHF photoconversion in an approach that combines broadband transient absorption with 100 fs pulses and two color experiments with sub-30 fs pulses. The former provides a clear identification of the transient states involved in the process, while the later allows us to determine precisely the kinetics of the process. Through the analysis of the coherent signal observed in addition we are able to identify the structural evolution of DHA directly after the application of the ultrashort pump pulse. 

The principle of the experiment is as follows a first pump pulse at 400 nm (Pump4oo) excites the electron donor (Pe) into its Si electronic state. After a variable time delay Ati (up to 1 ns) a conventionnal pump-probe measurement is performed at 530 nm (Pumps3o and Probes3o) on the ensuing transient species (Fig. lb). It allows us to determine the GSR dynamics of the Pe + cation upon excitation in its Do - Ds absorption band as a function of time delay after phototriggering the charge transfer reaction, i.e. as a function of the age of the transient. 

The only free parameter in the calculation of n is XB, the fraction of spins inverted by the pump pulse. The value of XB is determined from an independent PELDOR experiment of a standard biradical measured... 

Figure 1 Schematic representation of a time-resolved coherent Raman experiment, (a) The excitation of the vibrational level is accomplished by a two-photon process the laser (L) and Stokes (S) photons are represented by vertical arrows. The wave vectors of the two pump fields determine the wave vector of the coherent excitation, kv. (b) At a later time the coherent probing process involving again two photons takes place the probe pulse and the anti-Stokes scattering are denoted by subscripts P and A, respectively. The scattering signal emitted under phase-matching conditions is a measure of the coherent excitation at the probing time, (c) Four-photon interaction scheme for the generation of coherent anti-Stokes Raman scattering of the vibrational transition.

The widths of the spectral holes are comparable to the ones determined for ethanol solutions (78). For the latter samples we infer from model computations a size of the oligomers directly excited by the pump pulse of 4-6 (87), while in the isotopic mixture even longer associates may interact with the pump. Depending on the excitation frequency, a protonated species within an associate of 5-10 d6-ethanol molecules is estimated to be... 

The pump pulse in time-resolved pump-probe absorption spectroscopy is often linearly polarized, so photoexcitation generally creates an anisotropic distribution of excited molecules. In essence, the polarized light photoselects those molecules whose transition moments are nominally aligned with respect to the pump polarization vector (12,13). If the anisotropy generated by the pump pulse is probed on a time scale that is fast compared to the rotational motion of the probed transition, the measured anisotropy can be used to determine the angle between the pumped and probed transitions. Therefore, time-resolved polarized absorption spectroscopy can be used to acquire information related to molecular structure and structural dynamics. 

In an ACN sample jet 100 pm thick, we determined that the pump pulses can be focused no more tightly than 200 pm diameter. Table 1 lists typical parameters for the laser pulses used in this experiment, given that the probe pulse is focused to a slightly smaller diameter of 150 pm. Table 2 lists some properties of ACN relevant to experiments with C-H stretch pumping at 3000 cm 1. The fluence of the mid-IR pulses on the jet is 0.13 J/cm2,... 

TR methods were originally developed in om laboratories to study excited-state structures and dynamics of transition metal complexes such as Ru + (bpy)s and metaUoproteins. TR measurements rely on a pump-probe approach in which two separate laser pulses are used, one to excite the system and the other to probe the transient Raman spectrum. The time resolution of the experiment is determined by the width of the laser pulses (typically 7 ns for a Q-switched laser or as short as 1 ps for a mode-locked laser). The pulses are variably delayed with respect to one another to achieve time resolution, either by optically dela)dng the probe pulse with respect to the pump pulse or by electronically delaying two independently tunable lasers. Thus, two different approaches are required depending on the time scale of interest. The fastest timescale (from 10 to 10 s) requires optical delay to achieve sufficiently short separation between the pump and probe pulses. In such a scheme, the probe pulse is sent through a fixed path, but the pump pulse is sent through a variable path that can be scanned. Since hght travels about 1 ft per ns, a difference in pathlength of a few feet is sufficient. The second approach typically uses two Q-switched Nd YAG lasers that are electronically delayed with respect to one another, to access... 

Following the above-mentioned spectroscopic study by Johnson and co-workers [55], Neumark and co-workers [56] explored the ultrafast real-time dynamics that occur after excitation into the CTTS precursor states of I (water) [n — 4-6) by applying a recently developed novel method with ultimate time resolution, i.e., femtosecond photoelectron spectroscopy (FPES). In anion FPES, a size-selected anion is electronically excited with a femtosecond laser pulse (the pump), and a second femtosecond laser pulse (the probe) induces photodetachment of the excess electron, the kinetic energy of which is determined. The time-ordered series of the resultant PE spectra represents the time evolution of the anion excited state projected on to the neutral ground state. In the study of 1 -(water), 263 nm (4.71 eV) and 790 nm (1.57 eV) pulses of 100 fs duration were used as pump and probe pulses, respectively. The pump pulse is resonant with the CTTS bands for all the clusters examined. 

A simple, but often overlooked, control alternative is the gas-phase analog of the piston pump. In this scheme, the gas is added as a series of small gas-pulses of known volume and pressure. The pulse frequency sets the addition rate applying an equation-of-state to the volume of each pulse determines... 

Unlike Klyshko, who said that the advanced wave is a -function pulse, we consider it to be a continuous, partially incoherent wave. The duration of the advanced wave is in fact determined by the uncertainty of the photon arrival time measurement. With modern detectors, it amounts to at least tens of picoseconds. If the down-conversion experiment is performed in an ultrashort pulsed setting, this uncertainty substantially exceeds the pump pulse width, so the advanced wave can be considered continuous. On the other hand, if the pump laser is continuous, the situation is more complicated and the timing uncertainty must be taken into account more rigorously in order to determine the correct correlation function of the DFG wave and the density matrix of the conditional single photon. 

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