State-selective detection

Keil and co-workers (Dhamiasena et al [16]) have combined the crossed-beam teclmique with a state-selective detection teclmique to measure the angular distribution of HF products, in specific vibration-rotation states, from the F + Fl2 reaction. Individual states are detected by vibrational excitation with an infrared laser and detection of the deposited energy with a bolometer [30]. 

Recently, the state-selective detection of reaction products tluough infrared absorption on vibrational transitions has been achieved and applied to the study of HF products from the F + H2 reaction by Nesbitt and co-workers (Chapman et al [7]). The relatively low sensitivity for direct absorption has been circumvented by the use of a multi-pass absorption arrangement with a narrow-band tunable infrared laser and dual beam differential detection of the incident and transmission beams on matched detectors. A particular advantage of probing the products tluough absorption is that the absolute concentration of the product molecules in a given vibration-rotation state can be detenuined. 

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. 

Figure 2-1. A schematic diagram of the apparatus used to record photofragment angular distributions of complexes. An F-center laser is used to pump transitions in the parent complex which leads to dissociation. A second F-center laser is used as a probe to state selectively detect the fragments. The electrodes are used to orient the parent molecules prior to excitation.

Now we consider the desorption process combined with the molecular vibration of the adsorbate using a simple model. If we assume that a diatomic molecule sits on the on-top site with an upright geometry on the metal surface, we consider what knowledge is obtained from state-selective detection and the angular distribution of desorbed molecules. The vibrational modes characteristic of the adsorbate are frustrated rotation, which is the bending vibration of the molecule at the center of the adsorbate... 

Desorption from on-top species Buntin et al. [6] carried out observations on NO desorption from NO-saturated Pt(l 1 1) surfaces in detail, using X = 1064 (hoy = 1.17), 352 (2.33), and 355 nm (3.49 eY) at surface temperatures of 117 and 220 K. State-selective detection using the LIF method combined with the TOF measurement were used. In the present paragraph, the results observed at 220 K are described, because of the desorption from on-top species. Desorption occurs by the one-photon process, for which the desorption cross section is smaller at X = 1064 nm than those at X = 532 and 355 nm. Thus, the threshold energy for the NO desorption is regarded as <1.2 eV. 

The momentum p0 = J2mE depends on a form of the PES, in particular the excited-state PES, and on the residence time in the excited state, as supposed from Fig. 1. However, the form of the excited-state PES is unknown and further Ek cannot be estimated due to the effect of the energy dissipation in the excited state. On the other hand, both Et and Et delivered fromii can be experimentally measured by the state-selective detection method. Fortunately, p0 can be eliminated from both relations of Et = P2/2M = pl/2M and Et = L2/27 = (m2/M)2pl sin2 4 /2(jl. As a result, a simple relation between Et and Et is obtained,... 

Laser photodissociation of ketene at 230 nm has been investigated in molecular beams. The experimental rovibrational population distribution has been compared to predictions from phase-space theory for the channels leading to CO + CH2(a Ai) and CO + CH2(b Bi). The calculations are not compatible with the latter channel, suggesting that it does not contribute significantly to the dissociation process. The photodissociation of singlet ketene by two-step IR + UV excitation has been studied using state-selective detection of CH2 by laser-induced fluorescence, and the results compared with... 

We present the results of experimental studies of photon-negative ion interactions involving the dynamics of two electrons. Resonances associated with doubly excited states of Li and He" have been observed using laser photodetachment spectroscopy. Total and partial photodetachment cross sections have been investigated. In the former case, the residual atoms are detected irrespective of their excitation state, while in the latter case only those atoms in specific states are detected. This was achieved by the use of a state selective detection scheme based on the resonant ionization of the residual atoms. In addition, in the case of Li-photodetachment, the threshold behavior of the Li(2 P)+e-(ks) partial cross section has been used to accurately measure the electron affinity of Li. 

Fig. 1 Partial energy level diagram for the Li /Li systems showing the autodetaching decay of a doubly excited state of Li into different continua. Each of the three continuum channel is characterized by the state that the residual Li atom is left in following photodetachment via the doubly excited state. A particular decay channel is isolated by state selectively detecting the excited residual atom.

The state selective detection scheme based on RIS has been applied to isolate specific autodetaching channels for doubly excited states in both He" and Li. Here we present the results of recent measurements of the positions and widths of doubly excited states which appear as resonances in partial cross sections below certain excited state thresholds. The 1 s3s4s state in He, for example, lies just below the He(3 S) threshold. A Feshbach resonance associated with the autodetaching decay of this state was observed in both the He(ls2s S)+e(ks) and He(ls2p P)+e(kp) partial cross sectionsfhereafler labeled 2 Sks and 2 Pkp) [20]. Similarly, in the case of Li, resonances due to doubly excited states below the Li(42p) and Li(52p) thresholds were observed in the 32Skp partial cross section [21]. The state selective detection scheme has also been used to study the threshold behavior of the 2 Pks partial cross section. This technique enabled Haeffler et al. [22] to accurately measure the electron affinity of Li. 

When the state space spanned by /(0) includes more than 2 eigenstates, the dynamics in state space can begin to look very complicated. However, the concepts of bright state, dark state, state-selective detection, and... 

Figure B2.3.8. Energy-level schemes describing various optical methods for state-selectively detecting chemical reaction products left-hand side, laser-induced fluorescence (LIF) centre, resonance-enhanced multiphoton ionization (REMPI) and right-hand side, coherent anti-Stokes Raman spectroscopy (CARS). The ionization continuum is denoted by a shaded area. The dashed lines indicate virtual electronic states. Straight arrows indicate coherent radiation, while a wavy arrow denotes spontaneous emission.

Applications of these tunable VUV sources continue to be mostly in the detection of atoms and small molecules by laser-induced fluorescence in molecular beam scattering studies. Of particular importance has been the improved intensities available from Mg vapor, so that it will be possible, for example, to study the internal energy distributions in CO molecules following scattering from surfaces. This capability for both very sensitive and state-selective detection of small molecules will lead to important advances in our understanding of molecular interactions at surfaces. 

The product state distribution can be measured directly, for example by laser induced fluorescence (LIF) or resonance enhanced multiphoton ionisation (REMPI). Both of these techniques yield the quantum specific density of AB molecules that are created in the dissociation process. However, these methods can be applied only to a limited number of molecules. Whereas LIF is essentially restricted to a few diatomic molecules, REMPI allows in a few favoured cases also the state selective detection of larger molecules. 

Fig. 9.73 Schematic experimental setup for the one-atom maser with resonance cavity and state-selective detection of the Rydberg atoms [1294]...

Fig. 9.74 Experimental setup for experiments with the one-atom maser with three excitation lasers, cavity resonance microwave generation and a state-selective detection system...

Fig. 10.7 Schematic diagram of laser-induced chemical reactions with state-selective detection of the reaction products (a) by excitation of the reactants (b) by excitation of the collision pair (ABC)...

With the aim of quantitatively predicting the orientational order of rigid solutes of small dimensions dissolved in the nematic liquid crystal solvent, 4-n-pentyl-4 cyanobiphenyl (5CB), an atomistic molecular dynamics (MD) computer simulation has been applied. It is found that for the cases examined the alignment mechanism is dominated by steric and van der Waals dispersive forces. A computer simulation of the deuterium NMR spectra of molecules in a thin nematic cell has been carried out and the director distribution in the cell has been studied. An experiment for the direct estimation of an element of the order matrix from H NMR spectra of strongly dipolar coupled spins that is based on the multiple quantum spin state selected detection of single quantum transitions has been proposed. The experiment also enables obtaining nearly accurate starting dipolar... 

Kerman, A.J., Sage, J.M., Sainis, S., Bergeman, T., and DeMille, D., Production and state-selective detection of ultracold RbCs molecules, Phys. Rev. Lett., 92, 153001, 2004. 

Kubota, N., Hayashi, S. (2008) State selective detection of sputtered A1 neutrals by resonant laser ionization SNMS. Appl. Surf. ScL, 255,834-836. 

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