  Spectroscopic measurements of the rotational distribution of $^{84}$Sr and
$^{86}$Sr 5sns $^1S_0$ ultralong-range Rydberg molecular dimers created via
photoassociation in a cold gas are reported. The dimers are produced by
two-photon excitation via the 5s5p $^1P_1$ intermediate state. The use of
singlet states permits detailed study of the roles that the initial atom-atom
interaction, photon momentum transfer during Rydberg excitation, and sample
temperature play in determining the spectral lineshape and final dimer
rotational distribution. The results are in good agreement with the predictions
of a model that includes these effects. The present work further highlights the
sensitivity of ultralong-range Rydberg molecule formation to the state of the
initial cold gas.
