Tucker Carrington

Angular Momentum Distribution and Emission Spectrum of OH (2Σ+) in the Photodissociation of H2O

When water vapor absorbs light of wavelength less than 1350 A, an electronically excited OH fragment may be produced. This process has been observed, using wavelengths 1216, 1236, and 1302 A, and the rotational and vibrational excitation of the product OH 2Σ+ has been determined from its emission spectrum. Most of the energy beyond that necessary to produce electronically excited OH goes into rotation of that molecule. Roughly one‐third of the OH molecules have one quantum of vibrational energy, and in these molecules too, the highest energetically allowed rotational levels are strongly populated.Knowledge of the energy and rotational angular momentum of the OH allows estimation of the kinetic energy and orbital angular momentum of the H as it flies away from the center of mass. This is interpreted in terms of two‐body forces at large distances, with the most probable dissociation corresponding to an orbiting interaction.

Rotational, Vibrational, and Electronic Energy Transfer in the Fluorescence of Nitric Oxide

A sharp emission line of ionized cadmium from an electric discharge excites NO molecules to the K′=13 rotational level of the v′=1 vibrational level of the A 2Σ+ electronic state. Steady‐state fluorescence has been observed from this level and from neighboring levels populated by collisional interaction between the excited and added inert molecules. Cross sections have been measured in He, Ar, H2, N2, NO, and CO2 for electronic deactivation to the ground X2II state, for vibrational deactivation to the v′=0 level of the A 2Σ+ state, and for rotational exchange in the A 2Σ+ state. It has been established that rotational transitions having changes in rotational quantum number greater than unity take place with high probability, contrary to the optical selection rule.

Electronic Quenching of OH(2Σ+) in Flames and Its Significance in the Interpretation of Rotational Relaxation

The quenching of fluorescence of electronically excited OH(2Σ+) molecules in the burnt gases above a lean acetylene‐oxygen flame burning at a pressure of 3.5 mm Hg has been measured for resolved rotational lines. From these data the lifetime in this flame of the 2Σ+ state with respect to electronic quenching collisions is found to be 0.45×10—7 sec, about one‐tenth of the radiative lifetime of the free molecule. Quenching measurements as a function of pressure and composition give cross sections for quenching by O2, CO2, and H2O of 7, 16, and 35 A2. These values exceed by a factor of 10 or more the cross sections to be expected for simple exchange of rotational with translational energy. For argon the quenching cross section is less than 2 A2.The large cross sections for quenching by the flame gases, and the resultant short lifetime of electronically excited OH molecules in the flame mean that these molecules undergo a radiationless loss of electronic energy in collisions before they have time for either a...

Chemiluminescence of CH in the O +C2H2 Reaction : Rotational Relaxation and Quenching

Rotational distributions of CH (A 2Δ, v=0) were measured at pressures from 0.1 to 8.5 torr in the O+C2H2 reaction in the presence of large excess of N2, Ar, or He. The reaction N+NO→N2+O was used to produce O without O2. Under all conditions the distributions are found to be a superposition of two Boltzmann distributions, one at 1200°—1400°K characteristic of the process leading to the formation of excited CH (A 2Δ), and one which stays close to the temperature of the reactor. Assuming that the observed behavior of the rotational distributions is caused by collisions of excited CH with heat‐bath molecules and interpreting the fraction of molecules in this low‐temperature distribution as a measure of the extent of relaxation of the initial rotational distribution of excited CH, average relaxation rate constants are derived. With the collision partners mentioned above, 10 to 30 collisions are required for relaxation. A 14‐level phenomenological model including only transitions between neighboring rotational...

Transition Probabilities in Multilevel Systems: Calculation from Impulsive and Steady‐State Experiments

The study of collision‐induced transitions of molecules in a system of many energy levels is discussed in an analysis of time‐dependent and steady‐state experiments. These results are used in a discussion, with numerical examples, of the difficulties in calculating the transition probabilities from observed population distributions.An intimate conceptual and mathematical relationship is established between two types of experiments which are possible at present. One is an impulse, or time‐dependent experiment such as excitation with a fast flash lamp and snapshot observation after a known delay time. The other is a steady‐state experiment in which molecules are fed into the system (a gas or a surface) at a single energy level, for example, and undergo transitions among the available levels in competition with a first‐order removal process such as spontaneous radiation or desorption from a surface.The ideas summarized above are used to develop procedures for calculating transition probabilities from observe...

Wall effects on rotational population of OH2Σ+ in a microwave discharge☆

Abstract A study is presented of the rotational intensity distribution in the 2 Σ + - 2 Π electronic transition of OH radicals exeited in electrodeless discharges through argon containing about one-half percent water vapor, at a total pressure of 0.05 mm Hg. Two independent rotational distributions appear, their relative importance depending on the temperature of the wall of the discharge tube, the density of power dissipation in the discharge, and on the frequency of the exciting field. In the sensitivity of the spectrum to changes in discharge conditions, the wall plays a determining role.

Nonequilibrium Chemical Excitation and Chemical Pumping of Lasers

Chemical processes are reviewed which give rise to atomic or molecular products with spectroscopically observable nonequilibrium distributions among internal energy levels. Relevant theoretical considerations are reviewed, and for several types of reactions, experimental results are discussed in terms of specific examples. In several cases the existence of a population inversion seems clearly established, but it is by no means clear that the inversion density is sufficient for laser action. Certain features of molecular spectroscopy and collisional energy transfer processes bearing on the laser problem are discussed briefly.

Approach to Simple Exponential Decay in Vibrational Relaxation

When diatomic oscillators are dilutely dispersed in an inert heat bath, populations of the various vibrational levels in an initial nonequilibrium distribution relax toward thermal equilibrium with a time dependence that is a sum of terms like exp (λmt), where λ0=0, all other λm<0, and λm+1<λm. −1/λ1 ordinarily corresponds to the experimental relaxation time τ, since this term will dominate at long times. This work investigates cases in which the coefficient of the λ1 term, depending on the initial vibrational distribution and the heat‐bath temperature, may vanish or be so small that the final simple exponential decay corresponding to λ1 is never reached under experimental conditions.For harmonic oscillators, if the initial distribution has its first n moments equal to the corresponding moments of the equilibrium distribution, then the first n coefficients in the sum of exponentials will be zero. Furthermore, there will be one or more values of the heat‐bath temperature, independent of the initial distrib...