Short-pulse excitation

We show how one can image the amplitude and phase of bound, quasibound and continuum wavefunctions, using time-resolved and frequency-resolved fluorescence. The case of unpolarized rotating molecules is considered. Explicit formulae for the extraction of the angular and radial dependence of the excited-state wavepackets are developed. The procedure is demonstrated in Na2 for excited-state wavepackets created by ultra-short pulse excitations. 

The lifetime can be measured from a time-resolved experiment in which a very short pulse excitation is made, followed by measurement of the time-dependent intensity, as illustrated in Figure 8. 

A molecular fluorescence model is presented which is particularly appropriate for short pulse excitation. The frozen excitation model treats the two rotational levels which are directly excited by the laser as an isolated system with constant total number density. Consider the four level molecular model illustrated in Fig. 1. The four level model was solved by Berg and Shackleford (5 ) for the case where steady state is established throughout all molecular levels. Levels le and 2e are the single... 

If the K values are different, a large value of spin alignment could be obtained before the system reaches its steady state. This is accomplished experimentally by using pulsed excitation (32). The population ratio is simply equal to the ratio of the pumping rates, if the rate of pumping the zf level is much faster than its decay rate (a situation which is true at short times after short-pulsed excitation) (32), i.e.,... 

In pulse-echo-based techniques, the time of flight in a sample cannot be determined simply from the observation of the time span between adjacent echoes in the echo pattern if plane parallel transducers operated at resonant frequencies are employed. Transducers introduce substantial errors if the velocity is derived from such measurements, especially if relatively short samples are used. Various correction approaches have so far been developed in order to consider the influence of resonant transducers and the effects of diffraction [31-33]. The need for corrections can be avoided and a broad operational bandwidth obtained by using short pulses of duration equal to or shorter than the transduction [34] this requires a time resolution better than the transit time in the transducer. This short-pulse excitation (e.g. the maximum for a 10-MHz transducer is 50 ns) requires a high-power wide-band ultra-linear amplifier to ensure the detection of US signals with sufficient resolution under non-resonant conditions. 

Our purpose in mentioning negative ions here is to contrast their properties with those of systems which do exhibit Rydberg states. In particular, for negative ions, short pulse excitation is unable to produce narrow wavepackets of the kind described above, which arise by superposition of several Rydberg states. Thus, the response of negative ions to short pulses is markedly different from that of neutral atoms or positive ions, a matter which will be taken up again in chapter 9. 

Time-independent picture. The opposite extreme from short-pulse excitation involves the use of nearly monochromatic radiation. Practically, this means that the interaction between molecule and radiation field is of longer duration than Tnr. In this limit, the quantity measured is the absorption lineshape. It will be shown below that the linewidth observed in an energy-resolved experiment is related in a very simple way to the predissociation lifetime in the time-resolved experiment. 

A short pulse excites a coherent superposition of the eigenstates ip+ and ip which result from the interaction between the zero-order states ipi and ip2. This coherent superposition exhibits quantum beats at frequency and decay rate, respectively, wqb = (e+ - e )/fi and rQB/fi = (T+ + T )/ h,... 

As far as we are concerned, with ODMR detailed TRODMR measurements are required, including the use of short pulses of optical excitation and measurements of the spectral dependence of TRODMR signals. These measurements should be done on a-Si H samples prepared under various conditions. No evidence for localized exciton formation has been obtained in a-Si H. However, it might be worthwhile to search for such evidence by measuring TRODMR under short pulse excitation. 

If h(t) is the "ideal" response of a linear system to an impulse (i.e., an infinitely short pulse) excitation, and e(t) is an actual excitation waveform, then the observed response of the system is the convolution of h(t) with e(t) according to Equation 31. In general, the shape of the "ideal" response h(t) to an impulse excitation will not be obvious from the shape of the observed response f(t). 

Short pulse excitation, with At < h/de, which implies that At, < and AT < This corresponds to the foregoing broad-band coherent excitation case, where all the dynamic, periodic, or aperiodic properties of the molecular resonances are accounted for by the detection signal. 

Equation (1) shows that the SH signal should increase with increase of I, A, and T, but is eventually limited by surface optical breakdown. If we assume that the threshold energy for breakdown for a short-pulse excitation is (IT)tjj - 1 J/cm, then the maximum SH signal is given by ... 

As can be taken from the figure a localized wave packet is prepared in the short pulse excitation process. Its location is close to the potential barrier which separates the two product channels H -f- OD and D -f- Oif, i.e. the packet is found in the vicinity of the transition-state re on. Nevertheless parts of the initial wave packet have already moved towards the exit channels. Since the if-atom is lighter than the D-atom it... 

STRAFI-MRI is a slice-selective method since even very short pulses excite only a narrow portion of the sample Samoilenko s sensitive slice . 

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