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 ${\sigma_\rm{d}} = (5 \pm 2)\times 10^{-14};{\textrm{cm}}^2$ and $asd$ the vibrational quenching cross section for the (100) Sr–O stretching mode to be ${\sigma_q}$ $= (7\pm 2)\times {{10}^{-17}}{\rm c}{{{\rm m}}^{2}}$. The resulting ratio ${{\gamma }_{100}}={{\sigma }_{{\rm d}}}/{{\sigma }_{{\rm q}}}\sim 700$ is more than an order of magnitude smaller than for previously studied few-atom radicals (Au et al 2014 Phys. Rev. A 90 032703). We also determine the Franck–Condon factor for SrOH (${{\tilde{A}}^{2}}{\Pi_{1 / 2}}(100) \leftarrow {{\tilde{X}}^{2}}{{\Sigma }^{+}}(000)$) to be $(4.8\pm 0.8)\times {{10}^{-2}}$.
