The A1II - X1(Epsilon) electronic band of the CH(+) ion has been used as a probe of the physical and dynamical conditions of the ISM for 65 years. In spite of being one of the first molecular species observed in the ISM and the very large number of subsequent observations with large derived column densities, the pure rotational spectra of CH+ has remained elusive in both the laboratory and in the ISM as well. We report the first laboratory measurement of the pure rotation of the CH(+) ion and discuss the detection of CH-13(+) in the ISM. Also reported are the somewhat unexpected chemical conditions that resulted in laboratory production

Drouin, Brian J.

Pearson, John C.

NASA Technical Reports Server

NASA LAW, February 14-16, 2006, UNLV, Las Vegas
Laboratory Spectroscopy of CH+ and isotopic CH
J. C. Pearson & B. J. Drouin
Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 301-429,
Pasadena, CA 91109
John.C.Pearson@jpl.nasa.gov, Brian.J.Drouin@jpl.nasa.gov
ABSTRACT
The A1Π − X1Σ electronic band of the CH+ ion has been used as a probe
of the physical and dynamical conditions of the ISM for 65 years. In spite of
being one of the first molecular species observed in the ISM and the very large
number of subsequent observations with large derived column densities, the pure
rotational spectra of CH+ has remained elusive in both the laboratory and in the
ISM as well. We report the first laboratory measurement of the pure rotation
of the CH+ ion and discuss the detection of 13CH+ in the ISM. Also reported
are the somewhat unexpected chemical conditions that resulted in laboratory
production.
1. Introduction
The A1Π − X1Σ+ system of CH+ was first observed by Douglas & Herzberg (1941)
and later the 0-0 1-0 and 2-0 vibrational sub-bands were analyzed (Douglas & Herzberg
1942). The laboratory spectrum confirmed CH+ as a major constituent in interstellar clouds
(Adams 1942). In spite of the astronomical significance, subsequent laboratory work on CH+
has been relatively limited. The original data was extended to include the 0-1, 1-1, 2-1, 3-1
and 4-1 bands and Douglas & Morton (1960) extended the previous analysis to include the
0-1, 1-1, 2-1, 3-1 and 4-1 vibrational sub-bands. Later the 3-0 and 4-0 vibrational sub-bands
of the A1Π−X1Σ+ system and the 1∆−1 Π system and the 3Σ−3Π systems were observed
by Carre (1968). The 0-0, 0-1, 1-0, 1-1, 1-2, 1-3, 2-1 and 3-1 vibrational bands of the
A1Π−X1Σ+ system were remeasured at higher resolution and a complete set of molecular
constants for the ground state of CH+ were determined (Carrington & Ramsay 1981)
Laboratory investigations of isotopic CH+ have been similarly limited with all the avail-
able data coming from the electronic bands. The A1Π−X1Σ+ system was first tentatively
identified on the basis of interstellar spectra (Bortolot & Thaddeus 1969), which led to a
https://ntrs.nasa.gov/search.jsp?R=20060052439 2019-08-29T18:46:58+00:00Z
– 98 –
laboratory investigation of the 0-0, 1-0, 1-0, 2-1, 2-0 and 3-1 vibrational sub-bands (Antic´-
Jovanovic´ et al. 1983) confirming the identification. The spectrum of 13CH+ in the 0-0, 0-1,
1-0, 1-1, 2-0 and 2-1 bands A1Π − X1Σ+ system was revisited and recorded to higher J
values (Bembenek 1997a). The A1Π−X1Σ+ system of CD+ was first observed by Cisak &
Rytel (1971). The initial study was extended to the 0-0, 1-0, 2-0, 0-1, 2-1 and 3-1 vibrational
sub-bands (Antic´-Jovanovic´ et al. 1979). The 0-0 and 2-1 bands of CD+ were also measured
by Grieman et al. (1981). The most complete analysis of the A1Π− X1Σ+ system of CD+
including the 0-0, 1-0, 2-0, 0-1, 2-1, 3-1, 1-2 and 1-3 vibrational sub-bands was reported by
Bembenek et al. (1987). There is only one reported investigation of the A1Π−X1Σ+ system
in 13CD+ where the 0-0 and 1-0 vibrational sub-bands were recorded (Bembenek 1997b).
Reconciling the observed CH+ abundance in the interstellar medium with a chemical
production mechanism has been a long standing problem (e.g. Bates & Spitzer (1951);
Black & Dalgarno (1973); Black et al. (1975); Black (1998)). CH+ requires H2 for formation,
but is quickly destroyed by H2 as well. Radiative association of C
+ with H2 is to slow to
account for the destruction (Black et al. 1978) and the direct formation is endothermic and
therefore inconsistent with the cold gas optical line widths (Crane et al. 1995; Gredel 1997).
Heterodyne observations would clearly determine the origin of the CH+ in the interstellar
medium and would constrain the variety of mechanisms invoked to produce the large observed
column densities.
The J = 1 − 0 transition of CH+ is unfortunately close to the J = 5 − 3 Oxygen line
at 834.1 GHz. As a result, it cannot be directly observed from the ground and the only
astronomical detection was from ISO at too low spectral resolution to address the chemical
origins questions (Cernicharo et al. 1997). However, the 13CH+ J = 1−0 transition is shifted
far enough away from the oxygen line to facilitate detection.
2. Astronomical Detection of 13CH+
A search for 13CH+ based on the optical constants was carried out from the Caltech
Submillimeter Observatory towards the bright star forming region G10.6-0.4. Two features
were subsequently detected one in emission and one in absorption. In this source there are
three gas phase components along the line of site. The Star forming region G10.6-0.4, an
extended CO emission region and a deep HI self absorption region without CO have velocities
(vLSR) of -2 km/s, +29 km/s and +7 km/s, respectively. The emission line was assigned to
A state methanol 7+2 − 6
+
1 in the star forming region. However, the candidate
13CH+ line
could have a frequency of 830107, 830193 or 830132 MHz depending on the component of the
ISM where it originated (Falgarone et al. 2005). An analysis of the 13CH+ optical spectrum
(Bembenek 1997a) resulted in a predicted frequency of 830150 GHz but the error bars did
not preclude any of the possibilities.
– 99 –
3. Laboratory detection of CH+
Laboratory studies of CH+ have been problematic due to uncertainty in the chemical
production mechanism and technical challenges in generating microwaves at 835 GHz. The
microwave challenge was addressed by constructing a new direct synthesis spectrometer
(Drouin et al. 2005). Next, a new magnetically enhanced (De Lucia et al. 1983), variable
temperature discharge cell was constructed. As a commissioning experiment four transitions
of HCO+ in the ground and first excited vibrational were observed between 800 and 900 GHz.
This data is listed on Table 1 and compared to the frequencies in the JPL and CDMS line
catalogs. We had previously believed that HCO+ was sufficiently well studied for Herschel,
SOFIA and ALMA, but this is clearly not the case and it will be the subject of a separate
investigation.
Table 1: HCO+ transitions from this study, catalog entries and other experiments.
Ju-Jl V Measured JPL Catalog Cologne Database
9-8 0 802458.217(50) 802458.329(400)a 802457.970(133)b
10-9 0 891557.340(50) 891557.924(272)b 891557.038(203)b
9-8 1 802039.104(75) 802939.079(400)c 802038.613(413)b
9-8 1 805845.625(75) 805845.788(400)c 805845.545(464)b
avan den Heuvel & Dymanus (1982)
bCalculated
cBlake et al. (1987)
The initial series CH+ experiment started with acetylene and argon, but the only line
detected was from CH3CCH. A second series of experiment with improved cell cooling and
helium and argon was carried out. CH+ was discovered at 835078.950(75) GHz in a clean
cold cell with slow pumping speed at the start of the experiment. If the cell warmed up,
the 120,12 − 110,11 line of H
13
2 CO would appear at 835085.16 GHz and the CH
+ line would
disappear. The axial magnetic field and some helium were required for CH+ production.
Additionally the CH+ signal degraded as the experiment ran and coated the walls with
hydrocarbons. CH+ was also produced in a methane discharge under similar conditions
but at a lower methane flow rate since methane does not freeze out at the 90K cell wall
temperature. The best signal-to-noise always came from the highest voltage discharge that
was stable.
4. The Astronomical Origins of 13CH+
In order to determine the origin of the 13CH+ line two analyses of the new laboratory
frequency were carried out. First, the data from Carrington & Ramsay (1981) and Cer-
nicharo et al. (1997) were combined with the new laboratory frequency and a complete set
of Dunham constants (Dunham 1932) determined for CH+. Table 2 gives the constants for
– 100 –
CH+ determined from this analysis. The 3-1 band was excluded from the analysis since it
shows systematic deviations consistent with a perturbation of the upper state. The anal-
ysis improved by a tiny amount when the laboratory frequency was included. From these
parameters for 13CH+ were derived using isotopic ratios. This resulted in a 830131.4(2.4)
MHz frequency without accounting for Born-Oppenheimer breakdown (Watson 1973). A sec-
ond analysis was performed by fitting all the isotopic data including the Born-Oppenheimer
breakdown terms. This resulted in a 830166 MHz frequency with a large uncertainty. The
magnitude of the Born-Oppenheimer breakdown correction is known for CO (Wappelhorst
et al. 1997) and equates to +1.5 MHz. The correction from CH (Davidson et al. 2004) was
estimated on the basis of the B constant, which does not properly account for the vibra-
tional dependence and therefore over estimates the correction at +20 MHz. This leads to
the conclusion that the correction is small and positive. The only viable candidate from the
ISM is deep HI component without CO where the rest frequency is 830132(3) MHz.
Table 2: Derived Dunham constants for 12CH+.
Parameter Value (MHz)
Y10/2 42833117.(83)
Y20/4 -444387.7(244)
Y30/8 829.43(204)
Y01 425007.05( 79)
Y02 -41.3855(250)
Y03x10
3 1.923(215)
Y11/2 -7402.04(88)
Y12/2 0.3426(38)
Y21/4 16.544(93)
nlines 211
σred 1.518
σred
a 1.521
aσred without the laboratory line.
5. Conclusions
The rotational spectrum of CH+ has been observed for the first time in the laboratory
facilitating heterodyne astronomical observations from Herschel to constrain the chemical
origins of CH+. The 13CH+ frequency and observation should facilitate follow up work along
more lines of sight to confirm 13CH+ in the diffuse ISM and illuminate the non thermal
component that leads to its production. The very large A coefficient of CH+ 5.9 x 10−3s−1
should make it a major factor in the energy balance of the diffuse ISM providing an interesting
window into this still poorly understood gas phase.
This work was carried out by the Jet Propulsion Laboratory, California Institute of
– 101 –
Technology, under contract with the National Aeronautics and Space Administration. The
authors gratefully acknowledge the support of a NASA Astrophysics Research and Analysis
Program (APRA) grant
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Laboratory Spectroscopy of CH(+) and Isotopic CH

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