Photodissociation cross sections

Photodissociation cross sections tend to peak in the 10 cm range and hence are often given in Mb (megabam) units. 

For two Bom-Oppenlieimer surfaces (the ground state and a single electronic excited state), the total photodissociation cross section for the system to absorb a photon of energy ai, given that it is initially at a state x) with energy can be shown, by simple application of second-order perturbation theory, to be [89]... 

B. The Time-Dependent Formulation Total Integral Photodissociation Cross Section... 

The partial cross section gives the probability of absorbing light and producing a particular final product quantum state. The total photodissociation cross section is clearly given by the sum over all partial photodissociation cross sections ... 

Interaction of Photons with Molecules Photoabsorption Photoionization and Photodissociation Cross Sections... 

The absolute values of the photoabsorption, photoionization, and photodissociation cross sections are key quantities in investigating not only the interaction of photons with molecules but also the interaction of any high-energy charged particle with matter. The methods to measure these, the real-photon and virtual-photon methods, are described and compared with each other. An overview is presented of photoabsorption cross sections and photoionization quantum yields for normal alkanes, C H2 + 2 n = 1 ), as a function of the incident photon energy in the vacuum ultraviolet range and of the number of carbon atoms in the alkane molecule. Some future problems are also given. 

EXPERIMENTAL METHODS TO MEASURE THE PHOTOABSORPTION, PHOTOIONIZATION, AND PHOTODISSOCIATION CROSS SECTIONS... 

In particular, Shapiro and others calculated state-to-state photodissociation cross sections from vibrationally excited states of HCN and DCN [58], N2O [59], and O3 [60]. Eor instance, the detailed product-vibrational state distributions and absorption spectra of HCN(DCN) were compared [58]. These results were obtained employing a half-collision approximation, where the photodissociation could be depicted as consisting of two steps, that is, absorption of the photon and the dissociation, as well as an exact numerical integration of the coupled equations. In particular, it was predicted that large isotope effects can be obtained in certain regions of the spectrum by photodissociation of vibrationally excited molecules. 

B. Hiipper, B. Eckhardt, and V. Engel, Semiclassical photodissociation cross section for H2O, preprint (1996). 

Three types of measurements were performed in this study. First, photodissociation cross sections were measured, in which the total photofragment yield was measured as a function of dissociation photon energy. In these experiments, the electron signal generated by the microchannel plates is collected with a flat metal anode, so that only the total charge per laser pulse is measured. The beam block is 3 mm wide for these measurements. 

Photodissociation cross sections for CH2CHO and CD2CDO are shown in Fig. 1. Both spectra consist of sharp, extended vibrational progressions indicative of predissociation. A comparison of Fig. 1 with the LIF and absorption spectra [5, 7, 8] shows that we observe predissociation all the way to the origin of the B X band this is labeled peak 1 in Fig. 1. We observe photodissociation over the entire range of the absorption band. However, peaks 1-7 are the only peaks seen in the LIF spectrum. Peaks 1, 2,5, and 6 are particularly prominent in the LIF spectrum the latter three peaks are assigned... 

Figure 1. Photodissociation cross section of CH2CHO (top) and CD2CDO (bottom). Peaks 1—7 are the only features seen in the laser-induced fluorescence spectrum of CH2CHO (Ref. 8).

Photodissociation cross sections of ions are measured as a function of photon energy. Information is obtained on the photon-absorbing state(s) as well as on the predissociating or repulsive states formed on photon absorption. The method has not yet been employed for detection of metastable states in ion beams. 

The question arises how does one distinguish experimentally between these two types of photodissociation This question can be answered from consideration of the absorption spectrum. The predissociative state is bound, and, therefore, is characterized by a set of discrete levels. The indirect channel implies the appearance of resonant structure in the photodissociation cross section as a function of the frequency of the incident radiation. Hence, discrete structure in the absorption spectrum indicates the indirect nature of the photodissociation. For example, analysis of the absorption spectrum of C2N2 leads to the conclusion that the process C2N2 (C- -IIu)+ hv -+ CNCX rtj +CN(A II) at V = 164 nm is an indirect photodissociation process (8). 

The state-to-state differential photodissociation cross section is given by... 

Let us, for illustration purposes, consider the dissociation of a triatomic molecule as sketched in Figure 1.4. In an ideal experiment one would measure differential photodissociation cross sections with full specification of the initial state and full resolution of the final state,... 

In the following we will call the a u,n,j) partial photodissociation cross sections.t They are the cross sections for absorbing a photon with frequency u and producing the diatomic fragment in a particular vibrational-rotational state (n,j). Partial dissociation cross sections for several photolysis frequencies constitute the main body of experimental data and the comparison with theoretical results is based mainly on them. Summation over all product channels (n,j) yields the total photodissociation cross section or absorption cross section ... 

The calculation of absolute cross sections requires knowledge of the transition dipole function which, unfortunately, is rarely known. Therefore, all examples which we will discuss in this monograph are relative cross sections and the constant C will be mostly ignored in what follows. As stressed in Section 1.4, the total photodissociation cross section is the... 

H(Q) is the nuclear Hamiltonian in the corresponding electronic state at short distances it describes the motion of the complex and at large intermolecular separations it describes the free fragments. The matrix elements (3.1) are needed for the calculation of photodissociation cross sections. In this chapter we discuss numerically exact and approximate methods that are directly based on the solution of (3.2). The complementary time-dependent view follows in the next chapter. 

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