Oscillator Strengths and Predissociation Rates for Rydberg Transitions in 12C16O, 13C16O, and 13C18O Involving the E 1Pi, B 1Sigma+, and W 1Pi States

Abstract

One of the processes controlling the interstellar CO abundance and the ratio of its isotopologues is photodissociation. Accurate oscillator strengths and predissociation rates for Rydberg transitions are needed for modeling this process. We present results on absorption from the E ^1Pi-X ^1Sigma^+ (1-0) and B ^1Sigma^+-X ^1Sigma^+ (6-0) bands at 1051 and 1002 \AA, respectively, and the vibrational progression W ^1Pi-X ^1Sigma^+ (v'-0) bands with v' = 0 to 3 at 972, 956, 941, and 925 \AA, respectively. The corresponding spectra were acquired at the high resolution (R ~ 30,000) SU5 beam line at the Super ACO Synchrotron in Orsay, France. Spectra were obtained for the ^12C^16O, ^13C^16O, and ^13C^18O isotopologues. These represent the most complete set of measurements available. Comparison is made with earlier results, both empirical and theoretical. While earlier determinations of oscillator strengths based on absorption from synchrotron radiation tend to be somewhat smaller than ours, the suite of measurements from a variety of techniques agree for the most part considering the mutual uncertainties. For the bands studied here, their relative weakness, or their significant line widths arising from predissociation, minimizes potential problems from large optical depths at line center in absorption measurements. Predissociating line widths could generally be extracted from the spectra thanks to the profile simulations used in the analysis. In many cases, these simulations allowed us to consider e and f parity levels separately and to determine the dependence of the width on rotational quantum number, J. Our results are consistent with earlier determinations, especially the widths inferred from laser experiments.

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arXiv:astro-ph/0605186v1 6 May 2006
Oscillator Strengths and Predissociation Rates for Rydberg
Transitions in12C16O,13C16O, and13C18O Involving the E1Π,
B1Σ+, and W1Π States
M. Eidelsberg1, Y. Sheffer2, S.R. Federman2, J.L. Lemaire1,3, J.H. Fillion1,3, F. Rostas1,
and J. Ruiz4
ABSTRACT
One of the processes controlling the interstellar CO abundance and the ratio
of its isotopologues is photodissociation. Accurate oscillator strengths and predis-
sociation rates for Rydberg transitions are needed for modeling this process. We
present results on absorption from the E1Π−X1Σ+(1-0) and B1Σ+−X1Σ+
(6-0) bands at 1051 and 1002˚ A, respectively, and the vibrational progression
W
respectively. The corresponding spectra were acquired at the high resolution
(R ≈ 30,000) SU5 beam line at the Super ACO Synchrotron in Orsay, France.
Spectra were obtained for the12C16O,13C16O, and13C18O isotopologues. These
represent the most complete set of measurements available. Comparison is made
with earlier results, both empirical and theoretical. While earlier determinations
of oscillator strengths based on absorption from synchrotron radiation tend to
be somewhat smaller than ours, the suite of measurements from a variety of
techniques agree for the most part considering the mutual uncertainties. For the
bands studied here, their relative weakness, or their significant line widths arising
from predissociation, minimizes potential problems from large optical depths at
line center in absorption measurements. Predissociating line widths could gener-
ally be extracted from the spectra thanks to the profile simulations used in the
analysis. In many cases, these simulations allowed us to consider e and f parity
levels separately and to determine the dependence of the width on rotational
quantum number, J. Our results are consistent with earlier determinations, es-
pecially the widths inferred from laser experiments.
1Π − X
1Σ+(v′-0) bands with v′= 0 to 3 at 972, 956, 941, and 925˚ A,
1Observatoire de Paris-Meudon, LERMA UMR8112 du CNRS, France; michele.eidelsberg@obspm.fr;
jean-louis.lemaire@obspm.fr; Jean-Hugues.fillion@lamap.u-cergy.fr; francois.rostas@obspm.fr.
2Department
ysheffer@physics.utoledo.edu; steven.federman@utoledo.edu.
ofPhysicsandAstronomy,University ofToledo,Toledo,OH43606;
3Universit´ e de Cergy-Pontoise, LERMA UMR8112 du CNRS, France.
4Department of Applied Physics I, Universidad de M´ alaga, 29071-M´ alaga, Spain; jruiz@uma.es.

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Subject headings: ISM: molecules − molecular data − ultraviolet:ISM − tech-
niques: spectroscopic
1.Introduction
The photochemistry of CO plays an important role in photon dominated regions (PDRs)
of interstellar clouds, circumstellar shells surrounding AGB stars, planetary nebulae, circum-
stellar disks around young stars, and comets. Models of these regions (e.g., van Dishoeck &
Black 1988; Sternberg & Dalgarno 1995; Hollenbach & Tielens 1997, and references therein)
use observational data on CO transitions to extract physical conditions. The models rely on
oscillator strengths for dissociating transitions (e.g., Letzelter et al. 1987; Eidelsberg et al.
1991; Chan, Cooper, & Brion 1993; Federman et al. 2001; Sheffer, Federman, & Andersson
2003; Eidelsberg et al. 2004a). While a consensus is emerging for oscillator strengths per-
taining to many of the important electronic transitions (Federman et al. 2001; Eidelsberg et
al. 2004a), the large number of transitions leading to photodissociation warrants a careful as-
sessment of as many measurements as possible. Here we present results on the E1Π−X1Σ+
(1-0), B1Σ+− X1Σ+(6-0), and core excited W(A2Π+)1Π − X1Σ+(v′-0) bands with v′
= 0 to 3 for12C16O,13C16O, and13C18O acquired at the Super ACO synchrotron source in
Orsay, France.
These results are a continuation of our earlier efforts. Federman et al. (2001) studied
Rydberg transitions from the ground vibrational level in12C16O between 1150˚ A and about
1075˚ A. The limitation on the short wavelength end was the presence of LiF windows for the
gas cell. More recently, a system without windows based on differential pumping allowed
us to measure CO absorption from bands with wavelengths approaching 912˚ A, the Lyman
limit. Eidelsberg et al. (2004a) analyzed the strongly interacting bands [K − X (0-0),
L′−X (1-0), and L−X (0-0)] near 970˚ A in12C16O,13C16O, and13C18O. We now turn our
attention to the other transitions involving Rydberg states that were acquired during the
same synchrotron runs. The transitions discussed in the present paper are seen in interstellar
spectra (Sheffer et al. 2003) and are important for CO photodissociation (van Dishoeck &
Black 1988).
2.Experimental Details and Data Analysis
Since our spectra were acquired during the runs described by Eidelsberg et al. (2004a),
we only provide a brief overview here. The measurements were obtained on the Super ACO

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synchrotron ring using the high spectral resolution SU5 beam line (Nahon et al. 2001a, b).
The spectrometer consisted of a 2400 lines mm−1SiC grating blazed at 13 eV. The entrance
and exit slits were set at 35 or 25 µm, yielding respective instrumental widths of 30 or 20
m˚ A, as deduced from the analysis based on profile simulation described below. The vacuum
ultraviolet (VUV) light emerged from the spectrometer and passed through a windowless gas
cell; differential pumping maintained the ultra-high vacuum in the spectrometer and storage
ring. High purity CO gas (12C16O − Alphagaz, 99.997%;13C16O − Eurisotop, 99.1%13C,
99.95%16O;13C18O − Isotech, 98.8%13C, 94.9%18O) continuously flowed through the cell.
The pressure in the cell, typically 1 to 10 mTorr except for the weakest bands, was measured
with a capacitance gauge whose full scale was 10 Torr. The stability of the pressure was
checked by monitoring a cold cathode ionization gauge in the differentially pumped section
where typical pressures were 10−6Torr. The VUV flux exiting the gas cell was detected
with a photomultiplier tube (EMI 9558QB) placed after a sodium salicylate coated window
acting as a scintillator. The digitized signal from the PMT was recorded on a PC.
Oscillator strengths (f-values) were derived through syntheses of the measured absorp-
tion bands. As noted in Eidelsberg et al. (2004a), two independent codes were used to check
for consistency, where agreement at the 2% level was achieved. Previous analyses of ab-
sorption from Rydberg bands of CO with the Toledo-based code included the experimental
work of Federman et al. (2001) and the interstellar study of Sheffer et al. (2003). The code
developed in Meudon was used on A − X bands (Eidelsberg et al. 1999) and intersystem
transitions (Rostas et al. 2000). The synthetic spectra were based on tabulated spectroscopic
data (Eidelsberg et al. 1991). Each synthetic spectrum was adjusted to match the experi-
mental one in a non-linear least-squares fitting procedure with band oscillator strength, line
width (instrumental, thermal, and predissociation), and wavelength offset as free parame-
ters. Initial fits revealed that the thermal width was consistent with Doppler broadening at
295 K and was held constant in subsequent syntheses. Predissociation widths of 0.34 m˚ A
for the E −X (1-0) band in12C16O, 0.27 m˚ A in13C16O, and 0.18 m˚ A for13C18O were taken
from Ubachs, Velchev, and Cacciani (2000). (Hereafter we use the shorthand E1 because
absorption examined in the present paper always involves the ground vibrational level.) For
the W0, W2, and W3 bands, a J-dependent width had to be introduced in the synthetic
spectra in order to reproduce the observations, in keeping with the results of Drabbels et al.
(1993) and Eikema, Hogervorst, and Ubachs (1994).
The situation for the W2 bands required special care. According to Eikema et al. (1994),
the predissociation width is independent of J until J = 6, but dependent on J for J > 6
for e and f components in all isotopologues. We first determined the J-independent part
of the width from lines up to Q(6) and P(3), and then fitted the whole band to derive the
J-dependent part and the f-value.

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The first step in the analysis was to obtain an accurate measure of the CO column in
the differentially-pumped system. This was achieved through profile fits of the E0 band
near 1075˚ A for each isotopologue. Since the 3pπ (v′= 0) state is relatively isolated and
can be considered essentially free of perturbations, the band f-value was assumed to be
the same for the three isotopologues. Predissociation widths of 0.067 m˚ A for12C16O, 0.049
m˚ A for13C16O, and 0.047 m˚ A for13C18O (Cacciani et al. 1998) were used as input. The
pressure deduced from the fit to the E0 band was typically within 10% of the value given
by the pressure gauge. The deduced pressure was then adopted in syntheses of the bands
of primary interest scanned with the same set of conditions. For the high pressures needed
to measure absorption from the E1 band of13C18O – 24 to 58 mTorr, the values from the
pressure gauge were used.
Examples of our fits for E1, W0 and W3, and B6 bands appear in Figs. 1 to 3,
respectively. The ordering is based on predissociation width, from narrowest to broadest.
These figures reveal that several of the bands experience perturbations. In Fig. 1 accidental
predissociation at Je= 7 for13C16O (and12C16O) (Stark et al. 1992; Cacciani, Hogerworst,
& Ubachs 1995) gives rise to extra lines appearing with R(6) and P(8). In13C18O it appears
at Je= 1 and 5 (Ubachs et al. 2000) and gives rise to line shifts and broadening that are
barely visible in our spectra. These interactions arise from spin-orbit coupling between E1Π
(v = 1) and k3Π (v = 6). As for Fig. 3, the absorption bands at 1002.56˚ A in12C16O,
at 1002.81˚ A in13C16O, and at 1003.21˚ A in13C18O have been attributed to a transition
between the ground state X1Σ+v′′= 0 and a resonant state due to the strong coupling
between the 3sσB1Σ+Rydberg state and the repulsive D′ 1Σ+valence state (modeled by
Tchang-Brillet et al. 1992; Monnerville & Robbe 1994; Li et al. 1997; Andric et al. 2004).
This resonant state has been discussed in detail and identified by its Rydberg character to
the vibrational level v′= 6 of the B1Σ+state by Eidelsberg et al. (2004b). As predicted by
Andric et al. (2004), the broadening of these bands varies widely from one isotopologue to
another. Very weak and diffuse absorption bands involving the v′= 4 and 5 levels have been
observed recently (Baker 2005) below the avoided crossing of the diabatic potential curves.
B1Σ+v′= 6 is thus the first resonance state to be observed above the avoided crossing.
Two other processes affecting the f-values for B6 and W1 in13C18O are discussed in the
next section.
3. Results and Discussion
Our results are presented in Table 1 (f-values) and Table 2 (predissociation rates, kp)
along with values from earlier studies.The (FWHM) Lorentzian line widths (ΓL) that

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were determined from the spectra are linked to the corresponding upper level lifetimes
(τ) and transition rates (k) by the well known relation, ΓL = 1/(2πτ) = (1/2π)k (k =
ΓL(m˚ A) [1000/λ(˚ A)]21.885 × 1010s−1, where λ is the band origin for the transition). The
transition rate k is the sum of the radiative and predissociation parts (krand kp, respec-
tively). In the present experiments, the fact that ΓLcan be measured at all implies that kp
is considerably larger than kr.
Previous empirical f-values are based on synchrotron measurements (Eidelsberg et al.
1991; Stark et al. 1991, 1992, 1993; Yoshino et al. 1995), electron-energy-loss spectroscopy
(Chan et al.1993; Zhong et al.1997), and analysis of interstellar spectra (Sheffer et
al. 2003). As for the predissociation rates, an order of magnitude was first obtained by
Eidelsberg et al. (1991) from widths evaluated from absorption features recorded in low-
resolution synchrotron spectra. Subsequent efforts (Levelt, Ubachs, & Hogervorst 1992a,b;
Drabbels et al. 1993; Eikema et al. 1994; Komatsu et al. 1995) were based on high-resolution
line width measurements using laser techniques.
Table 1 lists the results on oscillator strength. Columns 1-3 give the transition, wave-
length of the band origin, and isotopologue, respectively. Our f-values appear in the fourth
column, and those from previous studies are shown in the remaining ones. We indicate
temperature for the measurements. For the transitions examined here, there generally is no
difference in f-values among isotopologues within the uncertainty of our measurements.
One exception involves the B6 band of13C18O, the f-value of which is significantly
smaller than that of the other isotopologues. It is to be noted that the 3dσ − X1Σ+(0-0)
band appears on the blue wing of the B6 band in13C18O, while it is not seen in the other
two isotopologues at the pressures used here (< 24 mTorr). As was shown in the detailed
study of Eidelsberg et al. (2004b), the 3dσ0 band borrows its intensity from the B6 band.
This effect is weak for the first two isotopologues, allowing this band to be seen only at high
column density. For the B6 band of13C18O, it appears that the mixing is stronger, leading
to similar intensities for the two bands. The pertinent value to consider is thus the sum of
the B6 and 3dσ0 oscillator strengths. This is about 0.0066, a value more in line with the
f-value for the other isotopologues. The second exception, W1 in13C18O, is so severe that
we are not able to derive an f-value. This band is strongly overlapped by another band of
unknown origin, whose profile does not resemble a πσ or σσ band. Since its intensity varies
with pressure as W1 does, the band probably arises from a valence state perturber. Thus
it is not possible to separate its contribution from that of W1. Finally, we note that the
f-values for the W bands are relatively strong, with the (2-0) band being the strongest. The
results for the W bands are consistent with the theoretical predictions of Cooper & Kirby
(1988) − i.e., f-values of about 10−2and Franck-Condon factors favoring the (2-0) band.