COOLING OF TRI-TERT-BUTYLBENZENE IONS - A TRPD THERMOMETRIC STUDY

Tri-tert-butylbenzene ions were heated by CO2 laser irradiation to an average internal energy of 0.63 eV in the ion cyclotron resonance ion trap. Their subsequent cooling by collisional and radiative processes was measured thermometrically using time-resolved photodissociation (T RPD) at 615 nm. Master equation modeling of ion heating and cooling wa s carried out to calculate the internal energy distribution of the ion s for deconvolution of the TRPD curves. Ion cooling in both low-pressu re (radiative cooling) and high-pressure (collisional cooling) regimes was indistinguishable from simple exponential energy decay. The rate constant for radiative cooling was measured as 1.06 +/- 0.15 s-1, and the rate constant for collisional cooling as 3.9 x 10(-10) CM3 molecul e-1 s-1. It was noted that the radiative cooling rate constant was alm ost identical with that for the closely related, but smaller, n-butylb enzene ion, and it was pointed out that, other things being equal, rad iative cooling rates should be independent of molecular size for molec ules initially at the same internal temperature.