Two-step excitation

Conventional associative ionization (AI) occurring at ambient temperature proceeds in two steps excitation of isolated atoms followed by molecular autoionization as the two atoms approach on excited molecular potentials. In sodium for example [44]... 

The excitation of the ground state H-atom product (n = 1) is made by the following two-step excitation scheme ... 

Because of the low energy of a ruby-laser photon (X = 6940 A A 1.8 eV), most of the photolysis experiments with ruby laser sources either use frequency doubling or proceed by two-step excitation or two-photon processes. 

Sorokin and Lankard illuminated cesium and rubidium vapors with light pulses from a dye laser pumped by a ruby giant-pulse laser, and obtained two-step excitation of Csj and Rbj molecules (which are always present in about 1 % concentration at atomic vapor pressures of 10" - 1 torr) jhe upper excited state is a repulsive one and dissociates into one excited atom and one ground-state atom. The resulting population inversion in the Ip level of Cs and the 6p level of Rb enables laser imission at 3.095 jum in helium-buffered cesium vapor and at 2.254 pm and 2.293 /zm in rubidium vapor. Measurements of line shape and frequency shift of the atomic... 

The incorporation of 1,3-dibutylalloxazine (DBA) as an electron carrier in the vesicle wall increased the accumulation rate of the reduced disodium 9,10-anthraqui-none-2,6-disulfate (AQDSH2) by 7 times. In the system of Fig. 6, the electron is pumped up in two steps at the inside as well as outside of the vesicle wall, however, since the photoreaction centers (surfactant ZnP) are the same for both the walls, the amount of energy acquired by the two steps excitation is equal to that by one step excitation. The incorporation of different photoreaction centers inside and... 

Polarization measurements can also be used to distinguish two-photon transitions from two-step excitations where the absorption of one photon produces an intermediate that may in turn absorb another photon. Frequently this intermediate is a triplet state. In liquid solutions SI = I should be found for such processes, since the absorption cross section will be given by the product of the average values for both steps and will therefore be independent of the state of polarization of the photons, if molecular rotation is rapid with respect to the intermediate and fluorescence lifetimes. 

Situation becomes niudi more complicated if we have a. two-step excitation. For example, the first laser excites transition 1 —> 2 and the second laser further excites transition 2 3. In this case for nondegenerated three level system we will have the following Liouville equations ... 

Andreoni et al. 49,83) investigated a selective photodamage of dye molecules bound to polynucleotides. The experiments were based on a time-delayed two-step photoionization the experimental set up is shown in Fig. 19. The two-step excitation was performed with a nitrogen laser (337 nm) and a dye laser (pumped by the nitrogen laser) at 430 nm. The selectivity arises from the fact that the lifetime of the first excited electronic state of the dye may be sensitive to the binding site (Chapter 4.2.2). Two complexes with different lifetimes Tj and t2 (tj < x2) are irradiated with two laser pulses (Fig. 20). The second pulse is delayed with rD against the first pulse. If xt < xD < t2 the second pulse (frequency v2) will find a smaller singlet-state... 

Interest in the study of higher energy states has increased in recent years and weak fluorescence has been observed from the S2 level of anthracene crystals using a two-step excitation approach (Katch et al.). Ultrafast relaxation from higher-lying excited states has been reported for several systems and the well-... 

Two-Step Excitation TG. When the dynamics of the excited states are investigated, it is preferable that two laser beams are used for the excitation to control the timing of the pump and probe. The first pump light creates the excited states and the second probe pulse probes the dynamics by creating the grating. However, since the probe process is the excitation... 

By combining the two-step excitation TG technique with the population grating (Section II.C), the rates and the quantum yield of reactions from higher excited states were determined for dimethyl-s-tetrazine [35] (Scheme 10). 

From these values, wavelength ranges to search for bound Rydberg series with field or collisional ionization or to search for autoionizing series converging to excited states of the ion were estimated for various parent levels that could be conveniently populated by one or two-step excitation. The threshold determinations reduced the search ranges for Rydberg levels to reasonable values. Scans were made from various parent levels until series were obtained. 

Emissions from both the and the previously unreported lli states of the IF molecule have been observed in the gas-phase reaction of L with F2 at low pressure a four-centre complex has been proposed as the reaction intermediate. A combined theoretical-experimental programme has been conducted to establish techniques for the study of excited-state transitions in Ij and IC1. Experimental techniques based on two-step excitation using two synchronized, tunable lasers have been developed, and successfully applied to excited-state fluorescence measurements on ICl. lodine(i) chloride adsorbed on silica gives the same Raman spectrum as that obtained from adsorbed l2. ... 

The second recent interesting application of lasers is concerned with the use of the narrow spectral bandwidth of the laser as a means of selectively exciting single components in mixtures, particularly in isotopic mixtures, thus achieving selective reaction and ultimately isotope separation. In order to obtain high selectivity, in general a two-step excitation process is required, but one-photon radiation of several species with i.r. lasers has been shown to lead to enhancement of reaction rates. Thus C02-laser-induced vibrational excitation of 03 leads to enhancement of reaction rates with SO (85), NO (86), and SF4-02 mixtures.222... 

Fig. 1.38 Field ionization of highly excited levels closely below the ionization limit (a) Potential diagram and (b) experimental arrangement for field ionization in a molecular beam. The photomultiplier monitors the LIF from the intermediate level populated in a two-step excitation process...

For pulsed laser excitation time of flight mass spectrometers are the optimum choice [495]. A schematic sketch of their design is shown in Fig. 4.37. By resonant two-step excitation and ionization in a molecular beam molecular ions are generated which are extracted by electric fields, pass a field-free region and are then acceler-... 

Fig. 5.22 Level scheme for two-step excitation of Rydberg levels of alkali atoms (a) and Rydberg series Na 3 3/2 measured by field ionization of the Rydberg states (b) [557]...

Fig. 5.27 Double excited planetary atoms (a) level diagram for two-step excitation of two electrons in the Ba atom with subsequent autoionization to Ba (b) experimental arrangement [574]...

Fig. 5.31 Experimental arrangement for the measurement of molecular Rydberg levels with two-step excitation in a molecular beam [582]...

For some spectroscopic problems it is necessary to use three lasers in order to populate molecular or atomic states that cannot be reached by two-step excitation. One example is the investigation of high-lying vibrational levels in excited electronic states, which give information about the interaction potential between excited atoms at large internuclear separations. This potential V R) may exhibit a barrier or hump, and the molecules in levels above the true dissociation energy V(R = 00) may tunnel through the potential barrier. Such a triple resonance scheme is illustrated in Fig. 5.42a for the Na2 molecule. A dye laser Li excites the selected level (v J )... 

Doppler-free two-photon spectroscopy or with two-step excitation (Sect. 5.4). For illustration, Fig. 8.5 illustrates pressure broadening and shifts of a rotational transition to a Rydberg level of the Li2 molecule measured with Doppler-free OODR polarization spectroscopy (Sect. 5.5) in a lithium/argon heat pipe [980], where the intermediate level B(v, J ) was pumped optically by a circularly polarized pump laser. For the chosen temperature and pressure conditions the argon is confined to the cooled outer parts of the heat pipe, and the center of the heat pipe contains pure lithium vapor (98 % Li atoms and 2 % Li2 molecules) with a total vapor pressure p(Li) = p(Ar) up to argon pressures of 0.7 mbar. The observed pressure broadening and shift in this range p < 0.7 mbar are therefore caused by Li + Li collisions. 

Fig. 1.41 (a) Relation between the incident light intensity and the frequency up-converted iight intensity, (b) Illustration of the two-step excitation and subsequent light emission, (c) Efficiency of the frequency up-conversion as a function of the incident light power density. For comparison, the efficiency of the second harmonic generation from a KDP crystal is tilso shown... 

In the early days of this type of two-step excitation, also known as optical-optical double resonance (OODR), one major aim was to be able to access molecular states (electronic states and vibrational levels) that were not normally accessible via singlephoton excitation. However, it was soon realized that the temporal independence of the two lasers would easily allow for the probing of dynamics in the system. Although this approach had only limited applicability (only dynamical processes of the order... 

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