PHOtofragment Fluorescence EXcitation

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... 

A detailed study of the predissociation channel (iii) from the C A state in HCN and DCN, which included analysis of the absorption and photofragment fluorescence excitation spectra, and measurements of the relative quantum yields and polarizations of the fluorescence following excitation into different vibronic levels, provides an example of the interrelation between photochemistry and spectroscopyThis was used to help characterise the nature of the excited electronic parent molecular states and provide information on the topography of the potential surfaces over which the predissociation proceeds. The photofragment fluorescence excitation spectra from HCN and DCN are displayed in Figs. 3.4. and 3.4.8 all the major vibronic bands can be attributed to progressions in the C state, and the CN(B - X) fluor-... 

Further information can be obtained from measurements of the polarization of the photofragment fluorescence excitation spectrum. The directions and magnitudes of the fluorescence polarization, which can be measured following photodissociation at any wavelength in the excitation spectrum, reflect the orientation of the electric dipole transition moment in the parent molecule and its lifetime with respect to pre-dissociation into the observed products . ... 

The CN(fi -> X) photofragment fluorescence excitation spectrum from the photodissociation of CH3CN qualitatively reproduces the absorption spectrum at wavelengths below the threshold, estimated at 146 nm A similar value... 

The nearest intermediate real state is A B and the anticipated final state is B Aj. On the other hand, the alignment of OH(A) fragnents generated through excitation into the structured features associated with I O(C B ) is dramatically reduced, and the reduction is in proportion to the relative contribution "structured continuum in the photofragment fluorescence excitation spearum. Predissociation of H O(C Bi) into OH(A) is only possible when out-qf-plane rotation about the a-inertial axis couples ih structured C state into the B state continmun [44]. This is just the motion necessary to tilt the molecular framewoik away from its initial alignment with respect to and hence reduce the final product rotational alignment - . ... 

Synchrotron radiation in the range 40.0—62.0nm was used by Wu et to dissociate N2. They measured the excitation function for producing photofragment fluorescence in the region 105.0—180.Onm, which was used to identify new N2 molecular states, and observed new states in the 21—22.5 eV... 

In this respect, there appears to be an alternation in the relative intensities of the progression in the bending mode (a 2) in the B Ce ) - X excitation spectrum of the photofragment fluorescence from NHjC/l With the revised vibrational num-... 

In the measurements of the PHOFEX spectrum, we scanned the VUV wavelength while the photofragment of S(]5) was monitored by exciting the S(3D )-S(lS) transition by UV laser light. Since only the fluorescence emitted from the S(3D]> fragments in the central region of the free-jet expansion was collected, the photoabsorption of ultracold (-5 K) OCS was selectively... 

The photolytic and probe pulses are colinear when they reach the sample. The photolytic pulse produces excited states and photofragments, and the probe pulse which follows closely behind must be used to analyse the concentration and/or the chemical nature of the transients. The major detection processes are known as laser-induced fluorescence (LIF) and multiphoton ionization (MPI). Transient absorptions can also be used in some cases, and this is similar to ps spectroscopy. 

LI Laser-induced Fluorescence. The probe wavelength Ap can be adjusted to excite one of the photofragments or the excited complex in the process of dissociation. Consider for instance the dissociation of a molecule AB according to... 

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.)... 

Let us consider some experimental studies. Developing practically the idea proposed as early as 1968 by van Brunt and Zare (see [176, 376] for a review), a number of authors [232, 328, 383] investigated fluorescence polarization on atomic photofragments excited in the process of photodissociation of the simplest diatomic molecules. Observation of linear polarization (of the order of V 0.05) in the fluorescence of Na(2P3/2) atoms after the photodissociation of Na2 molecules is reported in [328]. Detailed studies of photodissociation of K2 through the transition —> BlHu... 

Measurement of the degree of polarization of the fluorescence from an excited atomic fragment can give more information than f3 about the relative absorption amplitudes for excitation to different dissociative states. The alignment of photofragment emission can give information above the relative phases of the transition moments for photoexcitation of the parent molecule (3 samples only the squared transition moments), due to interference effects (Vigue,... 

Figure 8.23 Spectra illustrating various decay pathways for the H2 D1 , v = 8 level (a) photodissociation monitored by Lyman-a excitation of H atom photofragment, (b) VUV fluorescence detected LIF spectrum, (c) direct absorption, and (d) direct ionization to Hj" X2Ej v+ = 0. [(a) and (b) from Glass-Maujean, et al., 1987 (c) and (d) from Dehmer and Chupka, 1976 composite figure from Lefebvre-Brion, 1991.]...

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