Kinetic Investigation of the Time‐Resolved Atomic Fluorescence Ca(43P1 → 41S0) and Molecular Chemiluminescence CaI(A2Π, B2Σ+ → X2Σ+) Following the Pulse‐Dye Laser Generation of Ca(43PJ) in the Presence of CH3I
Authors: Basterrechea Francisco , Beitia Fernando , Castaño Fernando , Rayo Maria N. Sanchez , Carl Shaun , Husain David 
Published: July 2012

The collisional behaviour of the optically metastable, electronically excited calcium atom, Ca(4s4p(3P1)), 1.888 eV above its 4s2(1S0) electronic ground state, is investigated in the time‐resolved mode following its generation by pulsed dye‐laser excitation at elevated temperature from calcium vapour in the presence of CH3I and excess helium buffer gas in a slow flow system, kinetically equivalent to a static system. The removal of Ca(43P1) is monitored by time‐resolved atomic fluorescence at the resonance wavelength ( λ = 657.3 nm, Ca(43P1) → Ca(41S0) + hv), following rapid Boltzmann equilibration within the 43P1 spin‐orbit manifold using boxcar integration. Electronically excited CaI in the A2Π and B2Σ+ states, respectively 1.9372 (186.9 kJ mol−1) and 1.9466 eV (187.8 kJ mol−1) above the X2Σ+ ground state, was also monitored by time‐resolved molecular chemiluminescence of the A, BX systems under identical conditions to those employed for characterising the decay profiles of Ca(43P1). Both atomic and molecular chemiluminescence emissions showed exponential decay profiles, characterised by first‐order decay coefficients which were found to be equal under the same experimental conditions. CaI(A2II) and CaI(B2Σ+) are thus shown to arise from direct production on the collision of Ca(43P1) with CH3I where these processes are energetically favourable:

As with complementary measurements that have been reported on the collisional behaviour of Ca(43PJ) with CH3I under single‐collision conditions in molecular beams, where the AX and BX chemiluminescence of CaI was recorded, it was not possible to resolve the AX and BX profiles individually in view of spectroscopic overlap between these two close lying electronic states where vibrational population in higher levels of the A and B states at these elevated temperatures (900 K) also complicates the chemiluminescence spectra, as well as the spin‐orbit components in CaI(A2II1/2, 3/2). Time‐resolved chemiluminescence from CaI(A, BX) is hence employed the to monitor the global emission from these two electronically excited states. Thus, the A, BX emission can be used as an overall spectroscopic marker for Ca(43PJ) in the time‐domain and can be used to estimate a reaction rate constant for the overall collisional removal of Ca(43PJ) by CH3I of ca. 2 · 10−11 cm3 molecule−1 s−1 at T = 900 K though this must be viewed with caution on account of reaction of ground state Ca(41S0) with CH3I in the flow system prior to dye laser excitation. Broad band chemiluminescence is recorded for the Ca(43P1)CH3I system as well as laser‐induced fluorescence by excitation of the X2Σ+ state which is also generated on reaction of the excited atom. The principal conclusion of the present investigation in the time‐domain is the direct production of CaI(A2Π, B2Σ+) on the collision of Ca(43P1) with CH3I, in accord with complementary observations that have been made hitherto under single‐collision conditions and which are compared with the present investigation. The present results for CaI(A2Π, B2Σ+) are further compared with chemiluminescence measurements for analogous collisional processes for Ca(43P1) involving O, H, F, Cl and Br‐atom abstraction in the time‐domain and under molecular beam conditions where the complementary comparison is possible.