Dissociation of single rotational states

Stephenson, Casassa, and King 1988). An intriguing explanation of this spin selectivity based on the planarity of the transition state and the symmetry of the electronic wavefunction at the transition state has been provided by Alexander, Werner, and Dagdigian (1988) and Alexander, Werner, Hemmer, and Knowles (1990). 

As we emphasized at the beginning of Chapter 10, the rotational excitation of the fragment originates from the overall rotation of the parent molecule, the bending motion inside the parent molecule, and from the torque during the breakup in the excited electronic state. If the final state 

The necessary angular momentum coupling theory has been worked out by Balint-Kurti (1986). It includes  

1) The orbital angular momentum of the recoiling H atom with respect to OH. 

The final expression for the population of OH in a particular rotational state j (which, as a consequence of the electronic spin, is half-integer in this case) and in one of the four possible electronic fine-structure states, 2n1/2(A, A ) and 2n3/2( 4/, A ), which we designate by the index l = 1-4, is given by 

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While in the photodissociation of H2O through the AlB state the two possible A-doublets of OH(2n) are populated in a highly nonstatistical way, the two spin-orbit states, 0H( n ) and 0H( n3/2), are perfectly statistically populated. Unlike for the A-doublets there is a priori no geometrical reason to expect a difference in the spin-orbit states other than that given by the 2j + 1 statistical weighting factor. Since j = N + 1/2 for 2n3/2 and j = N — 1/2 for 2n1/2, the statistical weighting factor is (N + l)/N. Therefore, the population ratio 2n3/2 / 2n1/2 multiplied by N/(N-1-1) must be 1 for a statistical distribution as it is indeed measured in the bulk, in the beam, as well as in the dissociation of single rotational states of H2O (Andresen and Schinke 1987). The reason for the statistical... 

Only very few real state to state experiments have been done in photodissociation. The reason for that is very simple. The main problem of a real state to state experiment is to achieve product formation from single rotational states. In a gas at room temperature, many rotational states are populated and contribute to product formation. Even in most nozzle beams the state preparation is often incomplete, as will be demonstrated below. The main trick of the present state to state experiment is the preparation of single rotational states in vibrationally excited H2O and the subsequent dissociation at a wavelength where photolysis out of the vibrational ground state is impossible. 

Although the picture developed above describes the beam data extremely well we must point out that it fails to account for all the details observed if a single rotational state is dissociated. While the simple FC-type model predicts a preferential population of the 2n(A") state for all j, in the dissociation of single initial quantum states of H20(JA), on the other hand, one finds an oscillatory 2n(A") / 2n(A ) ratio (see Section 11.3). 

The upper part shows an H2O containing cell with a microphone and a phototube, which is irradiated by three different laser pulses to 1) prepare a single rotational state by IR excitation 2) to dissociate the molecule from this state and 3) to analyse the OH product. The process is explained in detail in the correlation diagram below. The lower right part gives a rough idea about the population of quantum states of H2O after IR excitation. 

Now the dissociation is done at 193nm. At this wavelength, absorption from vibrationally ground state H2O is negligible, whereas absorption from the vibrationally excited state (0,0,1) is very strong. If the OH products can be formed only from vibrationally excited states, the products have to come from the single rotational state of the asymmetric stretch mode. This is the idea of the experiment. 

A. Geers, J. Kappert, F. Temps, J.W. Wiebrecht, Preparation of single rotation-vibration states of CH30(C( E)) above the H-CH2O dissociation threshold by stimulated emission pumping. Ben Bunsenges. Phys. Chem. 94, 1219 (1990)... 

The trajectory studies in Ref. (78) were initiated by running a single-state trajectory on the Si surface of Cr(CO)6. Dissociation of a CO ligand was observed within 90 fs, and this ligand was noted to leave in a rotationally excited fashion, consistent with the measured rotational temperature observed in the similar W(CO)6 (103). Although the ejection of the ligand was the dominant nuclear motion, a subtle initialization of the in-plane symmetric L-M-L bending was also observed. 

The transition to the C B- state of H 0 was achieved by a two photon absorption of KrF laser light near 248 nm (32). The OH(A-X) fluorescence excitation spectrum in the 247.9-248.5 nm range follows the rotational structure of the C B -+ X A transition. However, (i) the OH(A-X) fluorescence spectrum produced by the two photon dissociation of H 0 has a maximum population at N = 14, while single photon absorption near 124 nm generates OH fluorescence spectrum with a maximum population at N = 20 (ii) only absorption to Ka= 1 (and not Ka= 0 where K is the rotational angular momentum about the a axis) of the ClB-L state predissociates into 0H(a2%) + H probably through the B A state. Apparently, the two-photon absorption near 248 nm predominantly populates the c b state, while the single photon process populates the B A near 124 nm. 

The rotational state distributions following the dissociation of a single initial state oscillate as a function of j, whereas the distributions obtained in the molecular beam or in the 300 K bulk are smooth functions. 

Resonantly enhanced two-photon dissociation of Na2 from a bound state of the. ground electronic state occurs [202] by initial excitation to an excited intermediate bound state Em,Jm, Mm). The latter is a superposition of states of the A1 1+ and b3Il electronic curves, a consequence of spin-orbit coupling. The continuum states reached in the two-photon excitation can have either a singlet or a triplet character, but, despite the multitude of electronic states involved in the computation reported J below, the predominant contributions to the products Na(3s) + Na(3p) and Na(3s) + Na(4s) are found to come from the 1 flg and 3 + electronic states, respectively. The resonant character of the two-photon excitation allows tire selection of a Single initial state from a thermal ensemble here results for vt = Ji — 0, where vt,./, denote the vibrational and rotational quantum numbers of the initial state, are stJjseussed. 

Continuous emission from NOa 359 and single vibronic level fluorescence from NOa 360 have been reported. The lifetimes of the 2Bl K > 0) states were measured, and the interesting result was obtained that the lifetime of the K = 4, N = 16 1 level was 36 [xs, substantially greater than that of the K = 0 levels. The increase in lifetime is attributed to Renner interaction of the and 2AX components of the linear 2 state. Rotational excitation has been shown to assist in the dissociation of NOa in the 249.1 nm system, but this is minor in extent compared with that observed in the 397.9 nm system. Yields of 0(4)) were reported.361 The photolysis of NOa has been further studied.362 In the report by Harteck et al., a two-photon excitation process was observed when a pulsed ruby laser was used for excitation. Thermal and photochemical reactions of NOa with butyraldehyde,363 other aldehyde-NO systems,364 and methylperoxyl radical-NOx reactions 365 have been discussed. 

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