The comparison of rotationally resolved infrared-vacuum ultraviolet-pulsed field ionization-photoelectron (IR-VUV-PFI-PE) depletion and IR-VUV-photoion measurements of C2H4 has illustrated the difference of these two methods and the rotational nature of the IR-VUV-PFI-PE depletion method. The IR-VUV-photoion spectra of trans-2-butene in the region 2910-3000 cm(-1) observed at different delays between the IR and VUV lasers are also presented to show that the IR-VUV-PI method is potentially useful for the study of intramolecular vibrational redistributions (IVRs). For short IVR lifetimes of vibrationally excited polyatomic neutrals, the IR-VUV-PFI-PE depletion method can serve as an alternative of the IR-VUV-PI scheme for rotationally resolved IR spectroscopy measurements. (c) 2008 Elsevier B. V. All rights reserved.