Collisional cooling in a quadrupole ion trap at sub-ambient temperatures

Quadrupole ion traps have been used successfully as ion storage devices for infrared multiphoton dissociation (IRMPD) experiments. Recently there has been interest in doing low temperature action spectroscopy experiments using ion traps as detectors for IRMPD spectroscopy. Cooling the trapping electrodes has the effect of cooling the trapped ions to the same internal temperature, because quadrupole ion traps operate at He bath gas pressures high enough to produce equilibrium ion internal energy distributions. It has previously been shown that collisions of the ions with the bath gas cool the ions’ internal vibrational modes at a rate that competes with IRMPD excitation at ambient temperatures. To investigate the implications of doing IRMPD in a quadrupole ion trap at lower trapping temperatures, the present study measures the collisional cooling rate constants at sub-ambient temperatures down to 25 K. The change in collisional cooling rate constant with trap temperature is correlated to the increased energy needed to dissociate the ions at these lower trap temperatures. It is found that both the energy needed to effect dissociation and the collisional cooling rate increase exponentially as trap temperature drops.