Halfwidth collision

Among other new methods, tunable laser absorption spectroscopy using infrared diode lasers offers prospects for improved accuracy and specificity in concentration measurements, when a line-of-sight technique is appropriate. The present paper discusses diode laser techniques as applied to a flat flame burner and to a room temperature absorption cell. The cell experiments are used to determine the absorption band strength which is needed to properly interpret high temperature experiments. Preliminary results are reported for CO concentration measurements in a flame, the fundamental band strength of CO at STP, collision halfwidths of CO under flame conditions, and the temperature dependence of CO and NO collision halfwidths in combustion gases. 

The collision halfwidth for a given transition is a function of temperature and the broadening species. In the present diode laser experiments, the temperature (and hence AVp) is known so that it is straightforward to infer values for the parameters a and AVq, and hence 2y, from the observed absorption linewidths. 

The temperature dependence of the collision halfwidth, 2y(T), is of fundamental and practical interest and has not previously been investigated at elevated temperatures. In the past, most determinations of collision halfwidth have been made near room temperature, and high-temperature values have been obtained by extrapolation, usually assuming a T 0>5 temperature dependence so that... 

An extensive series of room temperature experiments recently has been completed in our laboratory to determine the fundamental band strengths of CO and NO and to measure CO and NO collision halfwidths for N2, Ar and combustion gas broadening as a function of rotational and vibrational quantum number. Some preliminary results for CO are reported here. 

Experiments are currently in progress to measure CO and NO concentrations in a flat flame burner by diode laser spectroscopy. Comparative measurements are also being made using microprobe sampling with subsequent analysis by non-dispersive infrared and chemiluminescent techniques. Some preliminary laser absorption results for CO are reported here initial results for NO have been published separately (4). Also reported are initial data for collision halfwidths in combustion gases. 

Results showing the dependence of the CO collision halfwidth in combustion gases on the vibrational and rotational quantum numbers are shown in Figure 6. The data were obtained with a flame temperature of 1875 K and equivalence ratios in the range 1.2 - 1.4. Although too few data points are available for a detailed analysis, it is clear that 2y decreases with increasing m and that values for 2y are nearly equal (within 5%) for ground state and excited state transitions. 

The temperature dependence of the collision halfwidth for combustion gas broadening is also of interest. Results for specific transitions in CO and NO are given in Table I. In the... 

Varghese, P. L. and Hanson, R. K., "Diode Laser Measurements of CO Collision Halfwidths and Fundamental Band Strength at Room Temperature" to be published. 

In both cases (i. e. emission or absorption saturation) the halfwidth of this/Lamb dip is slightly dependent on laser power but mainly determined by the interaction time of the individual molecules with the standing light wave in the cavity. This time may be limited by the finite lifetimes rb of upper or lower states, by the average time l/aup between two disturbing collisions, or by the transit time Tt of the gas molecules across the laser beam. This last limitation becomes important at low pressures of the absorbing gas and for transitions between long-lived states (see Section IV.3). 

From the derivation in Sect. 3.1, one obtains a Lorentzian profile (3.9) with a halfwidth y = + y oi for the line broadened by inelastic collisions ... 

The preceding discussion has shown that both elastic and inelastic collisions cause spectral line broadening. The elastic collisions may additionally cause a line shift which depends on the potential curves E. (R) and E (R). This can be quantitatively seen from a model introduced by LINDHOLM [3.6], which treats the excited atom A as a damped oscillator which suffers collisions with particles B (atoms or molecules). In this model inelastic collisions damp the amplitude of the oscillation. This is described by introducing a damping constant such that the sum of radiative and col-lisional damping is represented by y = y + y qi From the derivation in Sect.3.1 one obtains for the line broadened by inelastic collisions a Lorentzian profile with halfwidth (3.38)... 

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