Determination of the Ion Vibrational Temperature

From the point of view of structure determination via vibrational spectroscopy, the goal of cryogenic cooling is to achieve vibrational temperatures sufficiently low such that hot bands from low frequency vibrational modes are not populated and thus do not complicate the spectra. As discussed in Sect. 1.2, for vibrational modes with frequencies on the order of 20 cm a vibrational temperature of 12 K would reduce the population of molecules in this state to 10% of the ground state. But how does one measure the internal temperature of a large molecule  

An alternative spectroscopic approach would be to determine the rotational temperature from the intensities in a rotationally resolved vibrational or electronic spectrum. In this case, even if one cannot resolve individual rovihrational transitions, one can still estimate the temperature by simulating the rotational contour of an individual vibronic band [120]. For this, one needs to know the rotational constants of the molecule and the direction of the transition moment however, even rough estimates of these quantities can lead to a reasonable temperature estimate. In measuring either Doppler widths or rotational band contours, the linewidth one obtains may contain a contribution from the finite lifetime of the molecule, determined by its intramolecular vibrational energy redistribution and/or dissociation rate if some type of photofragment spectroscopy is used, and this can make the temperature appear to be higher than it really is. 

Using this procedure. Red wine et al. obtained an upper limit for the temperature of 10-16 K for their 22-pole ion trap, consistent with what we measured in our 22-pole trap [46] as well as in a newly constructed cold octupole trap. In contrast, the 3-D quadrupole ion trap employed by Choi et al. attained temperatures of 45-54 K for protonated tyrosine [138], which is likely to reflect RF heating of the ions. 

Heating by the RF trapping field, which is more problematic for lower-order multipole traps, can result in the simation in which the translational temperature of the ions is different from that of the cold buffer gas. As Gerhch pointed out [20], if there is a sufficient number of collisions with the buffer gas, then it is the 

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