Line-mixing in the 106←000 overtone transition of HCN

Abstract

By using cavity ring-down spectroscopy (CRDS), we have obtained visible overtone absorption spectra of HCN which display a large collisional line-mixing effect in the proximity of the R branch band heads, for J∼18. We consider in detail the 106←000 (1=CN, 0=bend, 6=CH) parallel transition. The R branch profile was modeled using the modified-exponential-gap (MEG) and energy-corrected-sudden approximation (ECS) population transfer rate laws. We used the rates previously determined by Pine and Looney (PL) by fitting the self broadening coefficients measured for the Q branches of Π–Σ infrared perpendicular stretch–bend combination bands of HCN [J. Chem. Phys. 96, 1704 (1992)]. Contrary to what is found by these authors, in the present case the MEG law reproduces the R branch line-mixing satisfactorily, while the ECS model fails. This reflects an increasing propensity at higher J for collisional transitions with smaller ΔJ. Using the MEG law, we found we need to include, as had PL in their fits to the infrared Q branches, an empirical dephasing scale factor F∼0.6 for the coherence transfer rates to obtain a satisfactory simulation of the R band head. PL suggested that dephasing in the Q branch spectra are due to cross relaxation across l-type doublet levels of the Π state, but no such mechanism would be available in the present case. However, we have found that by using a 50/50 linear combination of the ECS and MEG rate laws, it is possible to fit our data even with F=1, which would imply no dephasing of coherence. We take this as a demonstration that the dephasing factor F cannot be reliably extracted from line-mixing studies alone but instead requires some independent source of information on the relative value for state to state inelastic collision rates.

Standard Reference Materials :

“…The large line dependence of the shifts may be due to collisional line mixing. HCN exhibits a large collisional line mixing effect, where collisions transfer coherence among transitions of overlapping lines [9]. This line mixing results in non-additivity of absorption lines as pressure broadening blends them together; the transitions can no longer be treated as independent absorption lines.…”

“…In Ref. [9], the authors speculate that strong line mixing could lead to an enhanced pressure shift. Clearly, the study of the HCN pressure shift mechanism is an interesting area for further research.…”

Temperature-dependent CO2 line mixing models using dual frequency comb absorption and phase spectroscopy up to 25 bar and 1000 K

“…As discussed in Section 3.3, when applied to infrared spectra, the MEG model only determines the real components of the off-diagonal relaxation matrix elements to within a constant of proportionality [6]. Here, we relate the relaxation matrix to the MEG-calculated transfer rates using an empirical parameter 𝐹 that simultaneously scales the population transfer rates and accounts for mixing between transitions in different branches [6,50,[53][54][55][56][57][58]. With this modification, the relaxation matrix is given by…”

“…For example, 𝐹 decreases from ~0.72 to ~0.55 as pressure is increased from 2 to 25 bar at a temperature of ~495 K. The values do not exhibit a clear relationship with gas density. Several past studies have noted the pressure dependence of the 𝐹 parameter, which is generally judged to be nonphysical and an indicator of the breakdown on the MEG approach for modeling collisional transfer rates [54][55][56]. As such, we assume pressure-averaged 𝐹 values in the MEG model for the remainder of this paper.…”

Collision-Assisted Spectroscopy of HCN above the Isomerization Barrier

“…Although not a global surface, considerable amount of data (see refs , , ) now indicate a high degree of success for the empirical force field of ref in predicting localized HCN ground-state energies.…”