Vibrational relaxation of strontium monohydroxide (SrOH) molecules in collisions with helium (He) at 2 K is studied.We find the diffusion cross section of SrOH at 2.2 K to be σ =±× − d (5 2) 10 cm 14 2 and the vibrational quenching cross sectionfor the (100) Sr–O stretching mode to be σq =±× − (7 2) 10 cm 17 2. The resulting ratio γ 100 = ∼ σ σ d q 700 is more than an order of magnitude smaller thanfor previously studiedfew-atom radicals (Au et al 2014 Phys. Rev.A90 032703 ).We also determine the Franck–Condonfactorfor SrOH ( Π Σ ← + A˜ (100) X˜ (000) 2 1 2 2 ) to be ± × − (4.8 0.8) 10 2.
