Multiphoton events

Fig. 7.62 Detection of multiphoton events at low detector gain. Left Correctly recorded signal, recorded at a detector gain of 10° and a count rate of 3-7 10" s. Centre and right. The same signal recorded at a detector gain of 10 and 10". The light intensity was increased until the PMT signal triggered the CFD.

Nevertheless, there is a way to reduce the classic pile-up. The light is distributed into several deteetors, or into several channels of a multianode PMT. Two photons arriving within the same signal period are then more likely to hit different detectors than the same one (Fig. 7.80, right). Pulses that appear at the outputs of different deteetors can easily be identified as multiphoton events. The recording of both photons in the TCSPC module can be suppressed and pile-up distortion can be avoided. 

Fig. 7.80 Multidetector operation reduces pile-up by recognising multiphoton events...

Luntz and co-workers have recently carried out an impressive study that follows in the spirit of the Eley-Rideal work.44 Specifically, laser-assisted recombination of N-atoms desorbing to form gas-phase N2 on Ru(0001) was investigated. Experimental measurements of state-selectively detected N2 recoiling from the surface recombination event were obtained using resonance enhanced multiphoton ionization and ion time-of-flight methods. In this way translational energy distributions of individual rovibrational states could be obtained experimentally. In addition, N2-vibrational population distributions could be derived. 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

A more sophisticated mode of LIE detection is the multiphoton-excitation (MPE) fluorescence [47], which is based on the simultaneous absorption of more than one photon in the same quantum event and uses special lasers, such as femtosecond mode-locked laser [48] or continuous wave laser [49], This mode of LIE detection allows mass detection limits at zeptomole level (1 zepto-mole=10 mol) due to exceptionally low detection background and extremely small detection volume, whereas detection sensitivity in concentration is comparable to that of traditional LIE detection modes. A further drawback is the poor suitability of MPE-fluorescence detection to the on-column detection configuration, which is frequently employed in conventional LIE detection. 

The interaction of keV particles with solids has been characterized by the measurement of the angle and energy distribution of sputtered secondary ions and neutrals. The results are compared to classical dynamics calculations of the ion impact event. Examples using secondary ions are given for clean Ni 001), Cu 001) reacted with 0>, Ni 001 and Ni 7 9 11 reacted with CO, and Agllll) reacted with benzene. The neutral Rh atoms desorbed from Rh 001 are characterized by multiphoton resonance ionizaton of these atoms after they have left the surface. 

The selection rules will be mentioned briefly here. In general, the process of multiphoton absorption is similar to that of single-photon absorption. The multiple photons are absorbed simultaneously to a real excited state in the same quantum event, where the energy of the transition corresponds to the sum of the energies of the incident photons. Thus selection rules for these transitions may be derived from the selection rules for one-photon transitions as they can be considered multiple one-photon transitions [20]. 

In the following, we will focus our discussion on reactions occurring in clusters in which one chromophore is surrounded by the solvent molecules the reactions occur when the chromophore is excited in its first excited state or ionized. The interest of using a, chromophore within the cluster is that multiphoton ionization can be used in connection with mass spectrometry. In this case, the ionization is a soft process and the spectroscopy of the cluster can give information on the size of the cluster, which is excited and responsible for the reactive event. This assigment can be difficult in other methods (electron bombardment) due to fragmentation processes associated with ionization. 

The technique of transient grating spectroscopy has been reviewed, with particular emphasis on its application to monitoring non-radiative deactivation. A unified theory of time-resolved fluorescence anisotropy and Stokes shift spectroscopy has appeared. A separate review has considered the chemical and photophysical events occurring from upper excited states as accessed by multiphoton absorption techniques. ... 

The dissociation has been studied photochemically by exciting the dipole active HN stretch with infrared multiphoton pumping and with one-photon local mode overtone excitation. From the discussions above, it is clear that we are primarily concerned in this chapter with the vibrational events which produce the activation of the N-N reaction coordinate and not with the electronic events which determine whether the curve crossing occurs. Hence, we have focused on the events on the ground electronic surface of the parent molecule which lead to an extension of the N-N bond to 3.5a . 

Solvent holes in neat cycloalkanes were generated by multiphoton ionization (3 x 4 eV or 2 x 5 eV) of the solvent at fluxes in excess of 0.01 J/cm [15]. In a typical experiment, the laser-induced dc conductivity was measured as a function of the delay time with resolution better than 3 ns. A similar setup was used to observe the dc conductivity in pulse radiolysis with fast 16 MeV electrons [14]. The decay kinetics of solvent holes in cyclohexane and decalins were consistent with the value of =1 for multiphoton laser ionization. For cyclohexane, a lower ratio of fh =0.5 was needed to account for the kinetics observed in pulse radiolysis. (Note that these ratios refer to the situation at ca. 10 ns after the ionization event the conductivity signal of the holes cannot be measured at earlier time). To be consistent with the observations, the simulations required a higher value for the mobility //jj of the... 

The Na source is placed between two identical samples. Two XP 2020 photomultipliers equipped with scintillators are attached directly to the two samples. The pulses from the photomultipliers are used as start and stop pulses for the TCSPC module. The pulses from PMT 2 are delayed by a few nanoseconds so that a stop pulse arrives after the corresponding start pulse. Eaeh y quantum generates a large number of photons in the scintillator. Therefore, the PMT pulses are multiphoton signals, and the time resolution can be better than the transit time spread of the PMTs. Moreover, the amplitudes of the photomultiplier pulses are proportional to the energy of the particle that caused the scintillation. Therefore the amplitudes can be used to distinguish between the 511 keV events of the positron decay and the 1.27 MeV events from the Na. The discriminator thresholds for start and stop are adjusted in a way that the stop channel sees all, the start channel only the larger Na events. The rate of the Na events is of the order of a few kHz or below. 

A major advance in the utility of laser spectroscopy came as a result of the development of multiphoton ionization MPI as a means of detection of multiphoton absorption by molecules [1]. The resonance encountered as the n-photon energy of a scanning laser becomes coincident with that of a molecular excited state is evidenced by a large increase in ionization rate. Since single ionization events can be detected with near unit efficiency, this results in a very sensitive means of detecting weak multiphoton absorption. MPI is a more widely applicable method than laser induced fluorescence since it can be used for non-emitting states. 

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