We have recently constructed an optothermal spectrometer, which is described elsewhere [16], to study the rotational and rovibrational spectra of molecules seeded in helium nanodroplets. While our goal is to use this technique to synthesize and characterize new free radicals and other unstable species, we began by studying the first acetylenic C-H overtone spectra of a series of terminal acetylene compounds. This affords us the possibility of addressing the problems of understanding the changes in moments of inertia induced by solvation as well as reliably predicting spectral shifts and lineshapes in this unusual and highly quantum environment. We hope to be able to use the He-solute and He-He pair potentials to predict `condensed phase' behavior of molecules dissolved in liquid helium. Furthermore we had previously used eigenstate-resolved molecular beam spectroscopy to determine the rates of IVR in these and similar molecules [21] and we wanted to determine whether solvation in liquid He would substantially change this rate.
The first molecule we studied was HCN, since at this energy it is not
expected to have IVR and is likely to have slow vibrational relaxation even
in He (due to its low density of vibrational states). Thus, this
molecule provides a `baseline' for the spectral width induced by
interaction with the He. Due to the low temperature and large rotational
constants of HCN, only an R(0) line could be detected. The line
proved to be asymmetric with a width of 
MHz, suggesting
the presence of two underlying lines. Studies of power dependence
showed that the
line could be saturated and allowed an estimate of the homogeneous
width of 
MHz to be made, showing that most of the linewidth
is due to inhomogeneous effects. Nauta and Miller have studied the
fundamental CH stretching spectrum of this molecule and found a much
narrower but also asymmetric line shape [22]. This implies
that the inclusion of the vibrational dependence of the interaction
between HCN and He will be required to explain these results.
FIG. 1. 
band (C-H stretch first overtone) of CH 
CCH
embedded in a 
He droplet. Due to the large distortion constant, the
R(3), R(4), and R(5) lines form a band head.
Other molecules studied in our laboratory include HCCD, HCCCN,
CH CCH, CF CCH, (CH ) CCCH, and (CH ) SiCCH.
Fig. 1 shows the spectrum of CH CCH with its
resolved P, Q, and R branches. Table I gives the
shifts in the vibrational origins and the observed full widths of
the spectroscopic lines.
TABLE I. band shifts and (range of) rotational
line widths for the molecules studied in this work.
For some of the molecules, the line widths
are J-dependent and we give a range of widths. When the gas
phase transition has an IVR-limited linewidth, Table I also
shows these FWHM values. While it is not possible from such data to
separate unambiguously the effects of the He on the IVR rate from
other sources of solvent induced dephasing, if we take the HCN
linewidth as representative of other effects, we can conclude that the
IVR rate is not qualitatively changed upon solvation in liquid He.
One exception may be propyne, which in the gas phase is known to have
a density of vibrational states at 
almost
sufficient to show isolated-molecule IVR on the 
ps time
scale [23]. The presence of the He solvent seems to push
this molecule over this threshold, probably because of relaxed energy and
angular momentum constraints in the condensed environment.
Work in other laboratories on very light rotors (HF [24],
H 
O [25], and NH [26]) indicates
that the line widths for these molecules are much broader than those we
have found in the series of CH-bearing molecules reported here. This
may reflect the very rapid rotation of these molecules. We note that
while Nauta and Miller have been able to detect the fundamental
vibrational spectrum of HCCH, and we have detected the C-H overtone spectrum of
HCCD, we have been unable to observe the overtone spectrum of HCCH.
This is probably due to very rapid solvent-induced vibrational
relaxation, with 
a reasonable candidate.
By irradiating the doped droplets with a tunable microwave field in a
microwave waveguide, we have also observed pure rotational transitions
of HCCCN [27]. To evaporate even a single He atom
requires an individual cluster to absorb 
microwave photons.
Based upon the relative strengths of the microwave and overtone
signals, we estimate that many thousands of microwave photons are
absorbed during the 
s time of interaction with
the microwave field. Based upon microwave double-resonance experiments
and experiments with a variable frequency of amplitude modulation, we
have estimated a rotational relaxation time of 
ns. The
microwave work also demonstrates that the lines are largely
inhomogeneously broadened, but in a dynamic sense. We estimate that
molecules change the quantum numbers responsible for the inhomogeneous
broadening at a rate one order of magnitude faster than the rate of
rotational relaxation. Microwave-IR double-resonance measurements,
performed in collaboration with Roger Miller [28], have
confirmed the dynamic nature of the broadening and have demonstrated
an interesting size dependence of the state-to-state
rotational relaxation.
The nature of the inhomogeneities that cause the broadening in the spectra is not clear at present. They are certainly not dominated by the droplet size distribution in our source. We currently believe that the most likely explanation is a coupling between the center-of-mass motion of the solute and its rotation. A thorough analysis of the motion of the impurity, assuming that the superfluid droplets provide no friction, has been recently given [29]. The long-range interactions between the solute and the `missing He' outside the cluster produce a potential that confines the solute to near the center of the droplet. Anisotropic long-range forces and a hydrodynamic coupling of center-of-mass motion with orientation lead to an inhomogeneous line broadening which is too small (in the case of HCN) to reproduce the experimental results. It is hoped that, by including the vibrational dependence of the He-HCN interaction and by providing a more realistic calculation to estimate the strength of the hydrodynamic coupling, the lineshape of HCN could be explained.
