Combining with electrospray ionization(ESI)mass spectrometry,infrared photodissociation(IRPD)spectroscopy is a powerful method to study structures of cluster ions in the gas phase.In this paper,infrared photodissociation spectrum of Pro_4H~+in the range of 2700–3600 cm 1was obtained experimentally.Both theoretically predicted spectra of the two most stable isomers of Pro4~(-1) and Pro4~(-2) obtained at the level of M062X/6-31+G(d,p)are in good consistent with the experimental results.The two isomers have similar structures and close energies.Both of them only consist of zwitterionic units,indicating the strong salt-bridged interactions inside the clusters.And the calculated collision cross section(ccs)of Pro4-1 is found to be very close to the experimental result previously reported. Combining with electrospray ionization (ESI) mass spectrometry, infrared photodissociation (IRPD) spectroscopy is a powerful method to study structures of cluster ions in the gas phase. In this paper, the infrared photodissociation spectrum of Pro_4H ~ + in the range of 2700-3600 cm 1 was obtained experimentally.Between theoretically predicted spectra of the two most stable isomers of Pro4-1 and Pro4-2 obtained at the level of M062X / 6-31 + G (d, p) are in good consistent with the experimental results. two ofomers have similar similar structures and close energies.Both of them only consist of zwitterionic units, indicating the strong salt-bridged interactions inside the clusters. And the calculated collision cross section (ccs) of Pro4-1 is found to be very close to the experimental result previously reported.