Rotational excitation of molecules

Femtosecond pulse trains offer another control knob to rotational excitation of molecules, namely, the control of its directionality. By applying a series of laser pulses, linearly polarized at an angle with respect to one another, either clockwise or counterclockwise rotation can be initiated in a molecular ensemble [27]. The effect has been experimentally demonstrated with a sequence of only two pulses [28], and longer pulse trains [29, 30]. The latter were dubbed chiral because of the directional rotation of light polarization from pulse to pulse shown in Figure 10.3a. Chiral pulse trains enable control of the sense of molecular rotation... 

S. Zhdanovich, C. Bloomquist, J. Floss, I. Sh. Averbukh, J. W. Hepburn, and V. Milner. Quantum resonances in selective rotational excitation of molecules with a sequence of ultrashort laser pulses. Phys. Rev. Lett., 109(4) 043003 (2012). 

Rotational excitation. The rotational excitation of molecules by electron impact has been studied less experimentally. At present there are no direct methods of measuring the cross sections of rotational transitions, and the latter are found by measuring the mobility of electrons in gases.228 So the available data are mostly theoretical. A favorable circumstance here is the fact that we are able to obtain sufficiently accurate analytical formulas for the cross sections of rotational excitation that do not require detailed knowledge of the molecule s structure.223... 

The Rotational Excitation of Molecules by Slow Electrons, Kazuo Takayanagi and Yukikazu Itikawa... 

Faubel M and Toennies J P 1977 Scattering studies of rotational and vibrational excitation of molecules Adv. Atom. Mol. Phys. 13 229... 

Molecular beams provide the answer. We first met molecular beams in Box 4.1, where we saw how a velocity selector is constructed. A molecular beam consists of a stream of molecules moving in the same direction with the same speed. A beam may be directed at a gaseous sample or into the path of a second beam, consisting of molecules of a second reactant. The molecules may react when the beams collide the experimenters can then detect the products of the collision and the direction at which the products emerge from the collision. They also use spectroscopic techniques to determine the vibrational and rotational excitation of the products. 

In order to see the effect of the rotational excitation of the parent H2O molecules on the OH vibrational state distribution, the experimental TOF spectrum of the H atom from photodissociation of a room temperature vapor H2O sample has also been measured with longer flight distance y 78 cm). By integrating each individual peak in the translational energy spectrum, the OH product vibrational distribution from H2O photodissociation at room temperature can be obtained. 

In microwave spectroscopy, the energy of the radiation lies in the range of fractions of a cm-1 through several cm 1 such energies are adequate to excite rotational motions of molecules but are not high enough to excite any but the weakest vibrations (e.g., those of weakly bound Van der Waals complexes). In rotational transitions, the electronic and vibrational states are thus left unchanged by the excitation process hence /ej = /ef and %Yl... 

J. Li, J. T. Bahns, and W. C. Stwalley. Scheme for state-selective formation of highly rotationally excited diatomic molecules. J. Chem. Phys., 112(14) 6255-6261 (2000). 

At elevated temperatures, ys falls off to near zero as a consequence of rotational excitation of the H2 molecules (which is here not accounted for), and the effect of the summation over many partial waves will further reduce the effects of symmetry. 

If we consider the collisions of two molecules (rather than atom + molecule, as above), the number of coupled channels is approximately the square of the number of accessible internal states of either molecule separately. Thus for rotational excitation of two hydrogen molecules near room temperature, Nc (jmax/2 + = 81 for Jmax = 4> and quantum calculations are... 

For rotational excitation of HC1 by Ar at room temperature, the maximum rotational angular momentum quantum number coupled during collision is about 12. The maximum number of coupled j,m states is Nc = (jmax + 1)(jmax + 2)/2 = since HC1 is a heterodiatomic molecule, and thus all states of the same total parity are coupled. With 91 channels, the quantum scattering calculations are feasible, but rather expensive. A further complication of the... 

Charge-transfer and dissociative-charge-transfer reactions of diatomic ions with various molecules that yield luminescence spectra are summarized in Table IV.B, part 3. In some of these, for example, the H2+ —N2 reaction, vibrational and rotational excitation have again been observed to accompany electronic excitation.155, 426 Molecular-ion reactions are generally accompanied by more extensive rotational excitation of the products than occurs with atomic-ion reactions.439... 

Coherent anti-Stokes Raman scattering (CARS) spectra of O2 (15) and H2 (16) have been recently obtained. This opens up a very important class of molecules to direct detection. It is now possible, with the right parent molecule, to study the dynamics of the molecular detachment process. It will be particularly interesting to see the dynamical results that will be obtained in the future on the VUV photodissociation of hydrocarbons, such as the. vibrational and rotational excitation of the H2 product. 

Three major sources contribute to the rotational excitation of the fragment molecule ... 

The vector of the electromagnetic field defines a well specified direction in the laboratory frame relative to which all other vectors relevant in photodissociation can be measured. This includes the transition dipole moment, fi, the recoil velocity of the fragments, v, and the angular momentum vector of the products, j. Vector correlations in photodissociation contain a wealth of information about the symmetry of the excited electronic state as well as the dynamics of the fragmentation. Section 11.4 gives a short introduction. Finally, we elucidate in Section 11.5 the correlation between the rotational excitation of the products if the parent molecule breaks up into two diatomic fragments. 

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... 

Chang, E.S. and Temkin, A. (1969). Rotational excitation of diatomic molecules by electron impact, Phys. Rev. Lett. 23, 399 103. 

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