Non-Boltzmann population

Although exit channel effects are capable of producing a range of non-Boltzmann population distributions, the wavelength dependence of the kinetic energy provides an indication that nonthermal activation is responsible for the fast component of the desorption signal. The activation mechanism responsible for this desorption process is not determined from these experiments, but will be re-addressed in section 4.6. 

The lifetimes measured by Miller and Lee probably correspond to the decay times of the SVL species with the rotational-state distribution close to the Boltzmann population, since the entire rotational band contour was excited and the entire emission band profile was observed. The lifetimes measured by Yeung and Moore probably correspond to the decay times of the SVL species with an appreciable non-Boltzmann population of the rotational states, since a small segment of the rotational band contour (probably only a few rotational lines) was excited. 

The same conclusions for the primary and secondary radicals can be achieved by steady-state (SS) irradiation in the ESR cavity termed SS ESR. SS ESR has several advantages and disadvantages as compared to TR ESR. First, SS ESR allows for detection of all free radicals, not necessarily only those in non-Boltzmann population (polarized). Second, SS ESR has ahigh sensitivity toward paramagnetic species in low concentration because it enables the application of field modulation. SS ESR spectra are presented in the well-known form as the first derivative of the signal. On the other... 

A common situation found in condensed phases under illumination is for all levels, except electronic levels, to be thermally equilibrated. Thus, under constant illumination, the sample is a mixture of thermally/vibrationally-equilibrated ground-state(s) with a very small, non-Boltzmann population of the excited electronic state, but which is itself thermally and vibrationally Boltzmann distributed. So the situation is similar to two non-equilibrated chemical species each of which is thermally equilibrated a thermally equilibrated ground-state, and a thermally equilibrated high energy excited-state. 

In order to probe the population of rotational levels of the desorbed NO, the time delay between the desorption laser and the LIF probe was flxed, and rotational excitation spectra were recorded. Fixed time delays of 9.0 or 3.0 is (corresponding to velocities of 415 and 1250m/s, respectively) were used to selectively interrogate desorbing molecules belonging primarily to either the thermal or non-Boltzmann component of the total desorbed flux. The desorption Hux in the thermal channel, probed at a time delay of 9.0 /is, was fitted well by a single Boltzmann distribution, with Tf = 200 20 K, somewhat lower than T .,. 

Fig. 11. Population distributions observed in the non-Boltzmann laser-induced desorption channel for NO/Pt(l 11). Populations in the higher (open cirdes) Fj and lower energy ( + ) F] spin-orbit states are distinguished. The heating laser wavelength was 532 nm.

The energy separation between the non-degenerate and degenerate states can be assessed by fitting the MCD intensity as a function of 1/T at a discrete wavelength to a Boltzmann population distribution for a two level system. 

A CARS experiment has recently been done to determine the amount of vibrational and rotational excitation that occurs in the O2 (a- -A) molecule when O3 is photodissociated (81,82). Valentini used two lasers, one at a fixed frequency (266 nm) and the other that is tunable at lower frequencies. The 266 nm laser light is used to dissociate O3, and the CARS spectrum of ( (a A), the photolysis product, is generated using both the fixed frequency and tunable lasers. The spectral resolution (0.8 cm l) is sufficient to resolve the rotational structure. Vibrational levels up to v" = 3 are seen. The even J states are more populated than the odd J states by some as yet unknown symmetry restrictions. Using a fixed frequency laser at 532 nm (83) to photolyze O3 and to obtain the products 0(3p) + 02(x3l g), a non-Boltzmann vibrational population up to v" = k (peaked at v" = 0) is observed from the CARS spectrum. The rotational population is also non-Boltzmann peaked at J=33, 35 33, 31 and 25 for v" = 0,1,2,3, and k, respectively. Most of the available energy, 65-67%, appears in translation 15-18% is in rotation and 17-18% is in vibration. A population inversion between v" = 2 and 3 is also observed. 

NO (v = 0, 2 = 1/2 and 3/2) are measured with a probe laser beam-sample distance of 1.65 mm at a delay time of 3.5 pis (velocity of 0.47 km/s), and shown in Fig. 14, which reveals a non-Boltzmann distribution. Furthermore, the two spin-orbit states exhibit inversion population, the 2 = 3/2 state being more populated. Since the RAIRS spectmm shows that the peak intensity at 1700 cm-1 decreases with laser irradiation, it is concluded that desorption of the on-top species occurs. 

The rotational energy distribution of desorbed NO from on-top species on Pt(l 1 1) at A. = 192 nm is observed to have a non-Boltzmann form, as shown in Fig. 14. Furthermore, the population in the two spin-orbit states is substantially inverted, since the population ratio of 2 = 1/2 and 3/2 is 1 2.2 in low J region. For desorption of hep hollow species at = 193 nm, on the other hand, the population ratio... 

The ground state CN(X2E+) from the reaction C(3P) + NO is produced in low vibrational (v = 0—3) and rotational (K — 0—35) states [420]. The vibrational population distribution decreases monotonically with increasing v, but is non-Boltzmann. The product rotational distributions... 

The reaction of CH3 with CF3 has been studied [643] using infrared chemiluminescence detection of the HF product, which is produced with v < 4( 0.13). The vibrational population distribution is non-Boltzmann despite HF being formed by unimolecular decomposition from a chemically activated CH3CF3 intermediate. It is suggested that the HF acquires excess energy as it separates from the CH2CF2. 

K, though for higher rotational levels the populations were higher than predicted by a Boltzmann model. The population ratio of the v = 1 level to the v = 0 represented a vibrational temperature of about 2200 K, which, though non-Boltzmann, is expected from the corona discharge source. Thus two vibrational levels of CN(B) could be studied relatively free from rotational congestion. 

Where an atom has a multiplet ground state, reaction may populate these sublevels with a non-Boltzmann distribution. This is difficult to observe since for light atoms the spin-orbit splitting is small and relaxation is rapid, and also because optical transitions between the components of the multiplet are strongly forbidden. Absorption measurements are possible but have scarcely been applied at all to this particular problem. 

Discussion of the these three methods is outside the scope of this book, but in later chapters we consider other methods for producing much less dramatic non-Boltzmann distributions. By using rf irradiation to alter spin populations, the nuclear Overhauser effect results in signal enhancement (Chapters 8 and 10). Several techniques use pulse sequences to transfer polarization from nuclei with large y to nuclei with small y in solids (Chapter 7) and liquids (Chapters 9 and 12), hence to provide significant signal enhancement. 

CIDEP (Chemically Induced Dynamic Electron Polarization) Non-Boltzmann electron spin state population produced in thermal or photochemical reactions, either from a combination of radical pairs (called radical-pair mechanism), or directly from the triplet state (called triplet mechanism), and detected by ESR spectroscope... 

The transitions between levels with non-equilibrium populations will be in the direction towards restoring the normal Boltzmann population their signal intensities will depend on the extent of non-equilibrium population. The observed effects are optimal for radical pairs with lifetimes in the nanosecond range. On a shorter time-scale, hyperfine induced intersystem crossing is negligible whereas on a longer time-scale the polarization decays owing to spin-lattice relaxation in the radicals. 

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