Densitometric Measurement of the Vibrational Relaxation of HCl and DCl in Shock Waves

The vibrational relaxation of HCl and DCl has been studied behind incident shock waves in the temperature range 700°–2100°K. Relaxation was followed by time‐resolved measurement of the postshock density gradient using a laser‐beam‐deflection technique. Hydrogen chloride is found to relax more rapidly than DCl. The relaxation times obtained at temperatures above 1000°K are summarized by the expressions ${\mathrm{(P\psi )}}_{\mathrm{HCl}}{\text{\hspace{0.17em}=\hspace{0.17em}(4.21\hspace{0.17em}±\hspace{0.17em}0.14)\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−7}}\exp {\text{[(64.0\hspace{0.17em}±\hspace{0.17em}5.7) (T}}^{\text{−1/3}}\text{\hspace{0.17em}−\hspace{0.17em}0.090)]}\mathrm{atm}·\sec \text{(1000°–2000°K)}$ , and ${\mathrm{(P\psi )}}_{\mathrm{DCl}}{\text{\hspace{0.17em}=\hspace{0.17em}(7.41\hspace{0.17em}±\hspace{0.17em}0.09)\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−7}}\exp {\text{[(77.7\hspace{0.17em}±\hspace{0.17em}2.1) (T}}^{\text{−1/3}}\text{\hspace{0.17em}−\hspace{0.17em}0.090)]}\mathrm{atm}·\sec \text{(1000°–2100°K)}$ . The values depart from Landau–Teller behavior at lower temperatures. These results are in serious disagreement with those of Borrell and with predictions of the SSH theory. However, they are in good agreement with the theory of Moore, suggesting that rotation–vibration energy transfer is important in the relaxation of HCl and DCl. Possible effects of strong dipole interactions are also discussed.