Photofragment yield spectra

A much clearer picture evolves when one decomposes the total spectrum into the partial photodissociation cross sections a(, n,j) for absorbing a photon with wavelength A and producing NO in a particular vibrational-rotational state with quantum numbers (n,j). Experimentally this is accomplished by measuring so-called photofragment yield spectra. The idea is, in principle, simple the NO product is probed by laser-induced fluorescence (LIF). However, instead of scanning the wavelength Alif of the probe laser (in order to determine the final rotational state distribution) one fixes Alif to a particular transition NO(2n, nj) —>... 

Fig. 7.14. Photofragment yield spectra for the photodissociation of C1NO through the T electronic state. The lower part depicts the total absorption spectrum crt<)t(A) measured at room temperature. The three separate spectra in the upper part correspond to the (unnormalized) partial photodissociation cross sections cr(, n,j) for producing NO in a particular vibrational state n as indicated. The rotational state varies between 1.5 and 4.5 in these three cases. The additional spectrum in the lower part is the n = 0 cross section originating from excitation in the So —> Si electronic band. Recall that the sum of all partial cross sections yields the total spectrum. Adapted from Qian, Ogai, Iwata, and Reisler (1990).

NO(2E, n j ) and monitors the total fluorescence from the excited electronic state of NO as a function of the photolysis wavelength A which was used to excite the C1NO parent molecule to the T state. If properly normalized, the photofragment yield spectra correspond to the partial photodissociation spectra discussed in Section 1.4.6. In view of (7.23) and Figure 7.6 the a(X,n,j) have the same resonance behavior as the total spectrum. The upper part of Figure 7.14 depicts three examples for various final vibrational states n the rotational state of NO varies between 1.5 and 4.5. (Because of the electronic spin of NO, the rotational quantum number j is half-integer in this case.)... 

Figure 12. Vibrational action spectra of V (OCO) in the OCO antisymmetric stretch region, (a) Spectrum obtained by monitoring depletion in the photofragment produced by irradiation at the vibronic origin at 15,801 cm The IR absorption near 2391.5 cm removes molecules from V[" = 0, leading to an 8% reduction in the fragment yield, (b) Spectrum obtained by monitoring enhancement in the VO+ photofragment signal as the IR laser is tuned, with the visible laser fixed at 15,777 cm (the Vj = 1 v" = 1 transition). The simulated spectrum gives a more precise value of the OCO antisymmetric stretch vibration in V" (OCO) of 2392.0 cm .

The photodissociation spectra of the Mg(L) complexes of ethyl isocyanate and ethyl isothiocyanate show some common photofragments. Aside from the ubiquitous formation of Mg+ (cf equation 39), both ethyl isocyanate and ethyl isothiocyanate yield products from attack of the N—C single bond (equations 57 and 58). The ethyl isothiocyanate complex also yields MgS via equation 59. The photodissociation spectrum of the ethyl thiocyanate isomer was also examined and gave the products shown in equations 60-62. Thus each isomer gives a unique ionic product [Mgs for ethyl isothiocyanate (equation 59) vs MgNC+ for ethyl thiocyanate (equation 62)] which allows their distinction. Finally, the Mg(ethyl isocyanate) +" complexes simply undergo solvent evaporation for n = 2 and 3 (cf equation 52). 

Other studies of the primary photochemistry of H2S at these short wavelengths are conspicuous by their absence. Lee et al. [118] measured the excitation spectrum for forming electronically excited SD(A) photofragments and found it to be a reasonably faithful reproduction of the DjS parent absorption spectrum for wavelengths shorter than about 151 nm. The fluorescence quantum yield was estimated at 4% [118] however, because... 

The CN(i4 - . T) photofragment fluorescence was first observed by Mele and Okabe following photolysis of HCN at 147.0 nm and 123.6 nm, and West has recorded the photofragment fluorescence excitation spectrum over the region occupied by the C v4 state the ectrum closely follows the absorption spectral profile. The quantum yield 0[CN(4)] varies between 0.10 and 0.18 over the wavelength range 150 nm - 130 nm Attempts have been made to monitor vibrational... 

---