Effective temperature of protonated tyrosine ions in a cold quadrupole ion trap

We obtained the ultraviolet photofragment spectrum of a protonated tyrosine ion using electrospray ionization quadrupole ion trap reflectron time-of-flight mass spectrometry to measure the effective temperature of the ion stored in the quadrupole ion trap (QIT) at ∼10 K. The rotational temperature was estimated to be 45–54 K from the rotational band contour analysis for the origin band of the S0–S1 transition, showing dependence on the buffer-gas pressure inside the QIT. We observed the rise of the rotational temperature with increasing the buffer-gas pressure, which could be attributed to the radio frequency heating effect of the buffer-gas collision. We also measured the vibrational temperature of the ion to be 50 K from the relative intensity of a hot vibronic band, taking its Franck–Condon factor into account. It turned out that the ion temperature rarely depends on the ion concentration of the electrosprayed solution under our experimental conditions. This study provides quantitative data indicating that a QIT is a good tool for cooling down the ions by the ‘buffer-gas cooling’ technique in laser spectroscopic studies.

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► We measured the effective temperatures of protonated tyrosine ions stored in the quadrupole ion trap at ∼10 K using laser spectroscopic techniques.
► The rotational temperature was estimated to be 45–54 K from the rotational band contour analysis, showing dependence on the buffer-gas pressure inside the ion trap.
► The rise of the rotational temperature with increasing the buffer-gas pressure was observed.
► The vibrational temperature was determined to be 50 K from the relative intensity of a hot vibronic band, taking its Franck–Condon factor into account.