Supplementary Material for O’Donnell et al.
Determination of pure overtone transition of CN-Ar
To determine the pure overtone transition of CN-Ar, (vCN, vs, nK) = (2,0,00) 
(0,0,00), electronic excitation spectra are recorded in the CN B-X (2,2) and CN B-X (2,0)
regions. Multiple features are observed in the B-X (2,2) spectral region which may be
correlated with transitions observed in the B-X (0,0) region1 (Figure S1, blue and red
traces). Two strong features are observed in the B-X (2,0) region at 30017.7 cm-1 and
30026.9 cm-1 which, when compared to the same features observed in the B-X (2,2)
region (at 25959.2 cm-1 and 25968.4 cm-1), allow for determination of the pure overtone
transition in the method described below.
To accurately obtain the value for the pure overtone transition, a frequency offset
is applied to the CN-Ar B-X (2,2) spectrum in steps of 0.10 cm-1 from 4058 cm-1 to
4060 cm-1. The CN-Ar B-X (2,0) and B-X (2,2) spectra are then subtracted and a
resulting residual trace is recorded for each value of the offset. The process is iterated for
each offset value and a set of residuals are recorded for each. The standard deviations of
the residual traces are then calculated and compared at each offset value. The minimum
was found to be at 4058.5 cm-1 with an uncertainty of 0.2 cm-1, corresponding to the
approximate bandwidth of the UV laser used to record the spectra.
1
Figure S1. CN-Ar B-X LIF spectra recorded in the CN B-X (0,0) region (blue), CN B-X
(2,2) region (red), and CN B-X (2,0) region (black). A schematic representation of the
electronic transitions is shown on the left-hand side of the figure. Spectral labels from
Ref. 1 are also shown above observed transitions observed in the CN B-X (0,0) region.
2
Reassignment of CN-H2 A-X spectrum
An alternative spectral assignment is proposed for the H2-CN A-X spectrum
recorded by Kaledin et al.2 The transition was originally simulated as a K=1 type
transition with a band origin of 12656.9 cm-1, corresponding to the position of the Qbranch of the perpendicular transition, and rotational constants, B=0.31 cm-1 and
B=0.28 cm-1. The transition is simulated here with the ground state rotational constant,
B=0.39 cm-1, derived from the IR spectrum, B=0.36 cm-1, and band origin of 12656.1
cm-1. The Q-branch now corresponds to what was previously assigned as a congested Pbranch. The transition that was assigned as Q(1) is resimulated as R(0) and the
corresponding higher energy peaks make up the R-branch. The P-branch is predicted to
be small relative to the Q- and R-branches and could be unobserved in the experimental
spectrum.
3
Figure S2. Alternative simulation of band contour for the A-X electronic transition of
H2-CN with rotational temperature of 4 K and Lorentzian linewidth of 0.25 cm-1.
Rotational assignments are denoted as ticks.
4
Figure S3: Correlation diagram depicting the energies of nK states for CN-Ar X (νCN=2)
at constant V10 (5.2 cm-1) as a function of V20. States of increasing n are indicated by line
color where black corresponds to n=0, red to n=1, and green to n=2. Different K states
are indicated by line type where the solid lines designate K=0, the long dashed lines,
K=1, and the short dashed lines, K=2. The blue reference line reflects the range of values
of V20 that reproduce the observed spacings based on experimental uncertainty, from 2.6
to 3.8 cm-1.
5
References
1
2
J. Han, M. C. Heaven, U. Schnupf, and M. H. Alexander, J. Chem. Phys. 128,
104308 (2008).
A. L. Kaledin and M. C. Heaven, Chem. Phys. Lett. 347, 199 (2001).
6