Photodissociation experiment

Figure B3.4.8. The correlation fimction c(t) = (ii1q vii(0) as a function of time for photodissociation m a collinear (or tliree-dimensional) polyatomic case. There are tliree relevant time scales T, which measures how rapidly the initial wavefimction dephases T2, which measures how long it takes this mitial wavefimction to regroup and which measures how long the wavefimction takes to Teak to other degrees of freedom. In practice, photodissociation experiments may yield spectra which are more blurred, and/or are not...

In this report we would like to discuss our results on the picosecond photodissociation experiments of the CO and O2 forms of a number of synthetic reversible oxygen carriers (1,2) and compared them to earlier picosecond absorption work on the same derivatives of the natural forms of hemoglobin. The latter work has provided us with a better understanding of the details of the photodissociation in terms of the sequential evolution of four photointermediates which were experimentally isolated, characterized, and kinetically analyzed (3,4). 

R. D. Levine The coherence that is being discussed by Profs. Troe and Zewail is due to a localized vibrational motion in the AB diatomic product of a photodissociation experiment ABC — AB + C. Such experiments have been done both for the isolated ABC molecule and for the molecule in an environment. As the fragments recede, effective coupling of the AB vibrational motion to the other degrees of freedom can rapidly destroy the localized nature of the vibrational excitation. 

Photodissociation experiments have become one of the most valuable tools in chemical physics for the purpose of understanding how excited electronic states couple to the dissociation continuum. These experiments, and the the-... 

While one might expect that the techniques developed for photodissociation studies of closed-shell molecules would be readily adaptable to free radicals, this is not the case. A successful photodissociation experiment often requires a very clean source for the radical of interest in order to minimize background problems associated with photodissociating other species in the experiment. Thus, molecular beam photofragment translation spectroscopy, which has been applied to innumerable closed-shell species, has been used successfully on only a handful of free radicals. With this problem in mind, we have developed a conceptually different experiment [4] in which a fast beam of radicals is generated by laser photodetachment of mass-selected negative ions. The radicals are photodissociated with a second laser, and the fragments are detected in coinci-... 

A similar approach was described by Hartmann et al. (114) for methylperoxy radicals in laboratory photodissociation experiments. The excimer laser pho-... 

Laser desorption Fourier transform mass spectrometry (LD-FTMS) results from a series of peptides and polymers are presented. Successful production of molecular ions of peptides with masses up to 2000 amu is demonstrated. The amount of structurally useful fragmentation diminishes rapidly with increasing mass. Preliminary results of laser photodissociation experiments in an attempt to increase the available structural information are also presented. The synthetic biopolymer poly(phenylalanine) is used as a model for higher molecular weight peptides and produces ions approaching m/z 4000. Current instrument resolution limits are demonstrated utilizing a polyethylene-glycol) polymer, with unit mass resolution obtainable to almost 4000 amu. 

For photodissociation experiments, a special probe was constructed to replace the 12.7 mm diameter direct insertion probe normally employed. It consists of a hollow stainless steel tube which has a 38 mm focal length quartz lens vacuum sealed at one end and an extended hollow probe tip at the other. The beam of a Lambda Physik excimer laser operating at 308 nm was passed through the probe and lens into the FTMS cell through a small hole in the center of the trap plate as shown in Figure 1. 

Figure 1. Diagram of Che experimental configuration employed for photodissociation experiments.

In this section we summarize briefly the various cross sections which can be measured in a photodissociation experiment, starting with the least resolved quantity, the absorption spectrum, up to the most detailed ones, final state resolved cross sections following the dissociation of a particular vibrational-rotational state of the parent molecule. We illustrate the hierarchy of possible measurements by an important example, the photo dissociation of H2O sketched in Figure 1.5.t For reviews of modern experimental methods see Leone (1982) and Ashfold and Baggott (1987), for example. 

The observation of resonances in a scattering experiment requires high resolution of the collision energy, which, however, is rather difficult to achieve in a conventional molecular beam machine. In a photodissociation experiment, on the other hand, the frequency of the light beam determines the collision energy in the upper state and hence the necessary energy resolution is much easier to accomplish. More crucial for the... 

Rotational excitation as a consequence of overall rotation of the parent molecule before the photon is absorbed does not reveal much dynamical information about the fragmentation process. It generally increases with the magnitude of the total angular momentum J and thus increases with the temperature of the molecular sample. In order to minimize the thermal effect and to isolate the dynamical aspects of photodissociation, experiments are preferably performed in a supersonic molecular beam whose rotational temperature is less than 50 K or so. Broadening of final rotational state distributions as a result of initial rotation of the parent molecule will be discussed at the end of this chapter. 

Kupriyanov, D.V., Sevastianov, B.N. and Vasyutinskii O.S. (1990). Polarization of thallium atoms produced in molecular photodissociation experiment and theory, Z. Phys. D-Atoms, Molecules and Clusters, 15, 105-115. 

An easy way to decide whether this is the case is to compare the bandwidth of the laser with the frequency width of the absorption spectrum of the molecule, where the latter is determined by (E, n deg i ,) 2. Typically, for the case of direct dissociation, the absorption spectrum extends over a few thousands of reciprocal centimeters (cm-1). In order to have a bandwidth broader than this, s(r) must, by Eq. (1.35), be as short as 1 to 5 femtoseconds (fs). Since most pulses used in real photodissociation experiments are much longer, Eq. (2.83) is not a valid description of many photodissociation experiments. 

Herschbach [58] noted a striking similarity between the recoil energy distribution of Cl atoms in the H + CI2 reaction and that observed in the photodissociation of CI2. This suggests that the electron attachment to the molecule is essentially a vertical process, hence he proposed the DIP extension to the model, which makes the AB repulsion after the electron jump analogous to that encountered in photodissociation experiments. This provided the necessary empirical basis for estimating the parameter of the repulsive interaction. All the mathematical expressions relevant to the model were given by Truhlar and Dixon [62]. Zare and co-workers extended the model to chemiluminescent reactions and a full account of the new model is given in Ref. [81]. It was used to predict successfully the product state distribution in the reaction Ca( So) -I- F2 —> CaF(B ) + F. 

Fifjure 6. Angular and kinetic energy distribution of the outgoing hydrogen atoms. Graphs a) and c) corresponds to the H + I exit channel, while b) and d) ndth the H + I channel. In the case a) and b) the dopant is placed in the second shell, while in the case c) and d) the dopant is on the surface, c) Simulation of the phorodissociation of the dopant on the surface (histogram) compared with a pick-up photodissociation experiment (line with error bars), f) Simulation of the photodissociation of the dopant in the sub-surface shell (histogram) compared with a pick-up photodissociation experiment (lino with error bars). 

Upper limits for the NO—NO and NO—Ar bond energies were given based on the observed data. They were found to be 6.3 1 and 1.4 + 0.3 kJ/mol, respectively. From bulk measurements it has been estimated that the bond energy of the dimer is about 8 kJ/mol. From photodissociation experiments done by Billingsley and Callear a value of 6.69 0.42 kJ/mol was obtained. The agreement between the upper limit obtained in this experiment and the value obtained by different methods means that almost no energy is deposited in vibrational degrees of freedom of the products. 

The surface coverage in both experiments was less than monomolecu-lar. Such photodissociation experiments with the sensitive EPR method have to be substantially improved by spectral efficiency measurements. 

The prototypical van der Waals molecule infrared photodissociation experiment was originally described by Klemperer in 1974 ( ) He proposed that infrared active constituents of weakly bonded clusters formed in molecular beams be excited with a laser. Since typical vibrational quanta are larger than typical van der Waals bond energies, subsequent intramolecular vibrational energy redistribution would lead to fragmentation that could be detected as beam loss with a mass spectrometer. 

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