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Unformatted text preview: 28Feb2011 Chemistry 21b Spectroscopy Lecture # 23 Electronic Spectroscopy and Non-Radiative Processes As was noted briefly in Lecture #20, the ultraviolet spectroscopy of formaldehyde (specifically the and n transitions) is complicated by the fact that at 3 eV or more of excitation the spectral lines are broadened by processes which limit the lifetime of the excited state. The energetics for the photochemical processes in H 2 CO are outlined in the figure below: 2 H CO CO + H 2 H + HCO H + HCO ? ? ? A 3 2 1 A 2 A 1 1 ( S) + ( ) 2 2 ( S) + ( ) 2 2 (29+(29 1 1 + g n * Figure 23.1 The photochemical energetics for formaldehyde photodissociation. A major question in how fast the various photodissociation processes can occur is the nature of the barriers involved. That is, what are the barriers to the photochemical production of H 2 + CO or H + HCO from formaldehyde? The energetics are easy to lay out, but if large activation energies are involved then it may take photons of considerably greater energy to drive the photochemistry. Other questions concern the nature of singlet-triplet coupling (i.e. is the 3 A 2 state involved in the decay of 1 A 2 ?), etc. Studies of the isolated benzene molecule in the late 1960s also raised a number of interesting questions. It was found, for example, that if the fluorescence yield (that is, the number of photons emitted/number of molecules excited) from the excitation into the first excited singlet state was examined as a function of pressure in a static cell, even at very low pressures only about 20% of the molecules excited actually emitted photons. Stranger still, if the fluorescence yield was examined as a function of vibrational energy content in the A state, the yield started at 20% and then dropped sharply toward zero above an excitation energy of 39,682 cm 1 . These results are summarized in Figure 23.2 below. The 180 fact that the fluorescence yield was well below zero, and changed rapidly with excitation, was cited as a breakdown of quantum mechanics by the research teams involved! 0.2 10 torr Ar 0.2 0.2 39,682 cm-1 Frequency f f 6 6 C H Figure 23.2 (Left) The fluorescence yield from the S 1 state of benzene as a function of background gas (in this case, Ar) pressure. Even at very low pressures the quantum yield is 1 [ J. Chem. Phys. 51 , 1982(1969)]. (Right) The fluorescence yield from the S 1 state of benzene as a function of internal energy in the S 1 state. The rapid drop in emission above a certain threshold energy was called the channel three problem, and working this problem out contributed greatly to our understanding of energy flow in isolated molecules [ J. Chem. Phys. 46 , 674(1967)]....
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