Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

Rate coefficient (cm 3 s -1 )
1e-10
1e-11
1e-12
−
−
1e-13
0 1000 2000 3000 4000
Temperature (K)
Figure 3.4: Rate constant (in cm 3 s −1 ) as a function of temperature for the vibrational relaxation of H2O(v2 =
1 → 0) by H2. QCT results are plotted as filled circles for T ≥ 1500 K (with error bars corresponding to 2 Monte-
Carlo standard deviations) and as arrows (lower limits) for T < 1500 K. The dotted curve denotes empirical
rates reported by (González-Alfonso et al. 2002 A&A, 386, 1074). The empty circle gives the experimental
value of (Zittel & Masturzo 1991 J. Chem. Phys., 95, 8005) at 295 K. The solid line corresponds to a standard
interpolation of the high temperature (T ≥ 1500 K) QCT results. Taken from Faure et al. (2005).
• Vibrational relaxation of H2O(v2 = 1) by H2
Classical calculations were carried out using our nine-dimensional H2O−H2 PES where only the bending
mode of water (first excited state at 1595 cm −1 above the ground state) was considered, i.e. all stretching
modes were neglected. The quasi-classical trajectory (QCT) method was employed as an alternative to
computationally impractical full close-coupling calculations. Our results, as presented in Fig. 3.4, have
shown that the rate constant for vibrational relaxation is one to two orders of magnitude greater than the
empirical prediction used by astrophysicists. Our high-temperature results (T > 1500 K) were also found
compatible with the single experimental point at 295 K. Moreover, we observed a significant rotational
enhancement of the vibrational rates, suggesting that standard quantum approximations (e.g. VCC-IOS)
might fail for molecule-molecule collision pairs with large rotational constants (Faure et al. 20005b).
• Rotational excitation of HC3N by para- and ortho-H2
Quantum close-coupling and classical calculations have been carried out at low temperatures (T < 50 K).
Our major result, as illustrated in Fig. 3.5, is the presence of strong quantum interferences in the rotational
rates. These interferences, which simply reflect the strong even anisotropy of the PES, are obviously absent
in our classical results and those of Green & Chapman for HC3N−He (1978). The ∆J = 2 propensity
rule was also shown to strengthen the inversion of the J = 1 rotational level of HC3N for H2 densities in
the range 10 3 -10 5 cm −3 , thus giving new insights to the HC3N astronomical masers. Another important
result is the complete absence of a para/ortho-H2 selectivity. This last result again reflects the particular,
non-multipolar, anisotropy of the PES.
• Methodological developments
The previous results have required original developments in the framework of quasi/semi-classical and
transition-state theories. In line with theories proposed a few years ago by Wiggins, Wiesenfeld and
colleagues (Wiesenfeld 2004; Wiesenfeld 2005), we have thus extended and generalized the concept of
transition states, widely used in the understanding of chemical reactivity, to rotationally inelastic collisions
(Wiesenfeld, Faure & Johann 2003). We have also reconsidered the semi-classical quantization of the rigid
asymmetric rotor (such as H2O) and we have shown that standard classical trajectories cannot be employed
to compute state-to-state cross sections in this case, owing to ambiguities in the assignment of the semiclassical
action to a particular quantum states (Faure & Wiesenfeld 2004). As a result, collisions involving
asymmetric top species does require a quantum treatment of rotation.
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