Infra-red emission arising from several room-temperature gas-phase reactions has previously been described by the authors in preliminary communications (Cashion & Polanyi 1958, 1959
a, b, c
). In the present work, details of this new technique are given. Spectra obtained from the systems H + Cl
2
, H + HCl, H + DCl and D + HCl are described. These consist of the resolved spectra of the HCl fundamental transitions (∆
v
= 1) in the ground electronic state, the partially resolved first overtones (∆
v
= 2) and, in one system, the unresolved second overtones (∆
v
= 3). The system H + Cl
2
gives rise to emission from all vibrational levels up to and including
v
= 6; the system H + HCl from all levels up to and including
v
= 7. A detailed examination of the spectra obtained from the systems H + HCl, H + DCl and D + HCl leads to the conclusion that these emissions arise from the formation of vibrationally excited HCl or DCl as the product of an association reaction between hydrogen atoms and chlorine atoms (in the presence of some ‘third body’,
M
). This result constitutes the first direct evidence for the view that association reactions lead to the formation of highly vibrating molecules (Polanyi 1959). Also consistent with this view is the observation made here that HCl or DCl acting as a third body in association reactions is not excited to levels higher than
v
= 1. The bulk of the emission observed from the system H + Cl
2
is believed to arise from the exchange reaction H + Cl
2
= HClꜛ
v
≼ 6 + Cl (where HClꜛ is vibrationally excited HCl in its ground electronic state). The vibrational distribution of HClꜛ in the system H + Cl
2
, under our experimental conditions, conforms approximately to a Boltzmann distribution for a vibrational temperature of 2700°K. From this observed distribution a calculation of the
initial
distribution is made, which would indicate that the HClꜛ are formed initially in all accessible vibrational levels, lower levels being favoured over higher. However, this result is based on the arbitrary assumption that vibrational-vibrational exchange between HClꜛ molecules is negligible. The distribution of HClꜛ among rotational levels of
v
= 1 in the system H + Cl
2
is definitely non-Boltzmann. The excess rotational energy over room temperature equilibrium energy, is shown to come from an even greater excess present in the HClꜛ as originally formed. The absolute intensity of the emission is calculated at
ca
. 0.005 W. It is estimated that roughly 1 to 10 % of the heat of reaction goes into vibrational excitation.
Collisional relaxation of apocarotenals: identifying the S* state with vibrationally excited molecules in the ground electronic state S0*