Infrared spectroscopy of the ν2 band of the water monomer and small water clusters (H2O)n=2,3,4 in helium droplets.

We have recorded infrared spectra in the frequency range of the ν2 band of water monomer and water clusters in superfluid helium droplets. In order to be able to map the chemically important fingerprint range, we have used an IR quantum cascade laser as a radiation source. We were able to observe three ro-vibrational transitions of the water monomer between 1590 and 1670 cm(-1). The lines were assigned to the 110 ← 101, 111 ← 000 and 212 ← 101 transitions of the ν2 vibration of H2O. Based upon the linewidths, we could deduce relaxation times of 1.9 to 4.2 ps for the monomer. Additional absorption bands could be assigned to ν2 vibrational bands of water clusters (H2O)n with n = 2, 3, 4. These experimental results are compared to theoretical calculations by Wang and Bowman which are reported in an accompanying paper [ref. 1, Y. Wang and J. M. Bowman, Phys. Chem. Chem. Phys., 2016, DOI: ]. We find a very good agreement of our results with both calculations and with previous results from gas phase cavity ringdown experiments.