Infrared cavity ringdown spectroscopy of water clusters: O– D stretching bands 

J. B. Paul, R. A. Provencal, C. Chapo, A. Petterson, and R. J. Saykally 

Department of Chemistry, University of California, Berkeley, California 94720 

Received 3 August 1998; accepted 9 September 1998 

The infrared O – D stretching spectrum of fully deuterated jet-cooled water clusters is reported.
Sequential red-shifts in the single donor O – D stretches, which characterize the cooperative effects
in the hydrogen bond network, were accurately measured for clusters up to ( D2O) 8 . Detailed
comparisons with corresponding data obtained for ( H2O) n clusters are presented. Additionally,
rotational analyses of two D2O dimer bands are presented. These measurements were made possible
by the advent of infrared cavity ringdown laser absorption spectroscopy IR-CRLAS using
Raman-shifted pulsed dye lasers, which creates many new opportunities for gas phase IR
spectroscopy.

© 1998 American Institute of Physics. S0021-9606 98 00847-2

Инфракрасная спектроскопия водяных кластеров

I. INTRODUCTION 

There is much current interest in the study of gaseous
water clusters by modern laser spectroscopy methods,1 – 3 as
such studies promise a route to an enhanced understanding of
the enigmatic condensed phase behavior of water.4 In addi-
tion to vibration-rotation tunneling VRT spectra that probe
cluster structures and intermolecular force elds,1 measure-
ments of the stretching and bending vibrations of the chemi-
cal O – H bonds are crucial because these directly probe the
cooperativity in the hydrogen bond network and the geomet-
ric distortion of the water monomer that accompanies H-
bond formation. Moreover, it is important to obtain data for
several isotopomers, as these provide exacting constraints on
the force eld determination. Previous gas phase IR
measurements5 – 9 have been restricted to the O – H stretch re-
gion 2.6 – 3.0 m because of the lack of suitable light
sources to extend the frequency coverage to other regions.
Here we report the application of the novel and more general
cavity ringdown laser absorption spectroscopy CRLAS
technique for the rst measurement of the O – D stretch vi-
brations in jet-cooled ( D2O n clusters, which provides new
insight into the nature of water cluster vibrations. The spec-
trometer used in this study operates continuously in the
2.5 – 7 m spectral region, with an average fractional absorp-
tion sensitivity of 1 – 2 ppm. 

Water cluster O – H stretch fundamentals were rst ob-
served in the gas phase by Lee and co-workers,6 using an
approach based on vibrationally predissociating the weakly
bound clusters with a pulsed, tunable OPO laser operating in
the 3 m region. Limitations in nonlinear crystal technology
that continue to exist today prevent these lasers from gener-
ating usable power at wavelengths longer than 4.0 m, thus
precluding studies of the corresponding stretching bands in
D2O clusters. Subsequently, many other gas phase studies of
water clusters have been reported.5,8,10,11 Other studies of
water clusters include matrix isolation experiments12,13 and
theoretical14,15 calculations. 

Studies of fully deuterated water clusters in the O – D
stretching region are now made possible by the advent of the
IR-CRLAS method, which has been described
previously.16,17 For the dimer, this has already produced an
improved understanding of the ground and excited state ac-
ceptor tunneling dynamics of the acceptor anti-symmetric
stretch.16 In the present work, we have recorded the discrete
absorption bands of ( D2O) n clusters ( n 9 ) , including a de-
tailed analysis of two additional dimer bands, which are
complicated by the numerous tunneling effects and are only
partially rotationally resolved. Additionally, a continuum ab-
sorption associated with clusters ranging in size from hun-
dreds to thousands of water molecules per cluster is dis-
cussed. These new results are compared with those for H2O
clusters in the gas phase, and D2O clusters in rare-gas matri-
ces. 

II. EXPERIMENT 

The IR-CRLAS apparatus used to conduct these experi-
ments has been discussed previously.16,18 Brie y, tunable in-
frared radiation is generated by Raman shifting a pulsed dye
laser Lambda Physik f13002e into the third-Stokes band
using a multi-pass cell containing 200 p.s.i. of H2 gas. The
bandwidth of the dye laser was switchable from 0.2 to 0.04
cm 1 by installing an intracavity etalon. After spectral lter-
ing, the laser light is aligned into a two mirror Ringdown
cavity. The light leaving the cavity is focused by a 10 cm
lens onto an LN2-cooled InSb detector. The resultant signal
is ampli ed, digitized, and transferred to a PC for real-time
tting to an exponential decay. The determined time constant
is divided into the cavity optical transit time to yield the per
pass fractional cavity intensity loss. 

The water clusters were generated in a pulsed supersonic
expansion. The helium carrier gas was bubbled through a
reservoir of room temperature water, and directed into a 4 in.
slit source19 contained within a Roots pumped vacuum
chamber. Various methods were used to systematically ad-
just the expansion conditions, including altering the source
stagnation pressure and limiting the amount of water in the
expansion with a needle valve, as discussed below. 

III. RESULTS 

Figure 1 shows a survey scan of the entire O – D stretch-
ing spectral region, with the band locations and most prob-
able spectral assignments given in Table I. As expected, the
spectrum resembles the O – H stretching spectrum observed
for H2O clusters under similar conditions. As such, many of
the features can be assigned by inspection. The bands sepa-
rate into characteristic absorptions regions, wherein the
free’’ O – D stretches are tightly grouped around 2700
cm 1, while the bonded’’ stretches exhibit large red-shifts,
extending hundreds of wave numbers toward lower fre-
quency. These red-shifts are a direct measure of the coopera-
tive effects within the hydrogen-bond network. 

The least red-shifted of the bonded stretches belongs to
the dimer. While H2O dimer stretch was found to be severely
lifetime broadened,11,18 the D2O cluster shows well-resolved
rotational structure of a parallel transition, permitting a de-
tailed analysis. This band system, occurring at 2632 cm 1
Fig. 3.2 , is the most intense of the observed ( D2O) 2 bands. 

Two main progressions can be identi ed, despite the signi -
cant spectral congestion caused by the tunneling splittings
and the parallel band structure. A close inspection reveals
that the weaker progression lacks transitions involving the
J 0 state, possesses a Q-branch, and exhibits a splitting in
the high-J rotational lines, indicating that it results from a
transition of a nearly symmetric rotor. Therefore,
we assign both of these progressions to the A 1 symmetry
component of the acceptor switching doublet. With this as-
signment, these progressions were t to a standard energy
level expression to derive molecular constants for the vibra-
tionally excited state. A simulation based on these constants
is also shown in Fig. 2, while the generated constants are
listed in Table II. 

FIG. 1. The O – D stretching spectrum of fully deuterated water clusters
taken under expansion conditions favoring the formation of small clusters
( n 10) but with a high degree of internal cooling see text for details .
From ab initio integrated absorption cross sections Ref. 22 , we estimate
the density of trimers and tetramers in the expansion 1 cm from the
ori ce to be 3 1013/ cm3, and 7 1012/ cm3, respectively. 

TABLE I. Measured band positions and assignments for ( D2O) n clusters. 

Shift relative to D2O monomer anti-symmetric stretch 2789 cm 1 . 

FIG. 2. IR-CRLAS spectrum of the ( D2O) 2 -bonded O – D stretch. Below is
a simulation based on the molecular constants listed in Table II and a rota-
tional temperature of 10 K. The sticks represent D2O monomer transitions,
which were used to frequency calibrate the data. 

TABLE II. Determine molecular constants of the ( D2O) 2 bonded and free
O – H stretches. All constants given in cm 1.

A third, even weaker progression can also be identi ed
within this band system, which presumably corresponds to
the other acceptor switching component ( A 2 symmetry .
This tunneling motion, which is the only one of the three
presently accepted feasible tunneling pathways that does not
require breaking a hydrogen bond, is also the only one that
commonly produces large enough splittings to be resolved at
the present resolution. Assuming that the splittings from the
other two tunneling motions donor – acceptor interchange
and bifurcation are unresolved, the intensity ratio of the ac-
ceptor tunneling components is expected to be 2:1 based on
nuclear spin statistics, which agrees well with the present
results. Unfortunately, this progression is too weak to permit
rotational analysis.