A&A 516, L4 (2010)

LETTER TO THE EDITOR

Submillimeter-wave spectrum of CH₂D⁺

T. Amano

Department of Chemistry and Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada

Received 6 May 2010 / Accepted 17 May 2010

Abstract
Aims. Recently a tentative identification of CH₂D⁺ in interstellar space has been reported. To facilitate astronomical identifications, laboratory measurements of precise rest frequencies for the rotational lines of CH₂D⁺ should be carried out.
Methods. A submillimeter-wave spectrometer is used for detection of CH₂D⁺. The CH₂D⁺ ion is generated in an extended negative glow discharge operated at liquid nitrogen temperature. The optimum gas mixture is found to be CH₄ (3 mTorr), CD₄ (1 mTorr), and H₂ (2 mTorr) in helium buffer.
Results. Four rotational lines have been detected in the frequency range of 280-890 GHz. The measured frequencies agree very well within a MHz with the predictions given by Rösslein et al. from the infrared spectra.
Conclusions. Two rotational lines of this ion have been tentatively identified toward Ori IRc2. The rest frequencies obtained here should facilitate identifications and analysis of astronomical spectra.

Key words: molecular data - methods: laboratory - ISM: molecules - submillimeter: ISM - radio lines: ISM

1 Introduction

In interstellar carbon chemistry, CH₃⁺ is likely to be one of the more abundant molecular ions, and its deuterated species, CH₂D⁺, is thought to play an important role in deuterium fractionation in warmer interstellar clouds (Turner 2001; Roueff et al. 2007; Parise et al. 2009). However, because CH₃⁺ is a symmetric planarmolecule and as a result has no permanent dipole moment, it is almost impossible to detect this species by radio astronomical observations. Its deuterated species, CH₂D⁺ and CHD₂⁺, possess the dipole moment, so the rotational lines should be observable. Rösslein et al. (1991) and Jagod et al. (1992) observed the infrared spectra of these deuterated species by using a high-resolution infrared spectroscopic technique with difference frequency radiation as a radiation source. Demuynck and coworkers (see for example Demuynck 1994) tried to observe CH₂D⁺ rotational lines in submillimeter-wave region in an extended negative glow discharge with no success. Wootten & Turner (2008) searched for CH₂D⁺ in several molecular sources without definite identifications. More recently Lis et al. (2009) reported a tentative identification of CH₂D⁺ toward Ori IRc2. Thus laboratory measurements of the rest frequencies for this ion is urgently needed. This letter reports the first laboratory identification and the precise measurements of the submillimeter-wave lines of CH₂D⁺.

2 Experimental procedure and results

A backward-wave oscillator (BWO) based submillimeter-wave spectrometer (Amano & Maeda 2000; Amano & Hirao 2005) was used in conjunction with an extended negative glow electrical discharge cell (De Lucia et al. 1983). With three BWO oscillators, the frequency range of 280-890 GHz is covered with two frequency gaps, 530-560 GHz and 750-770 GHz. The frequency of submillimeter-wave radiation was phase-locked to harmonics of millimeter-wave radiation (80-110 GHz) from a Gunn oscillator that could be controlled by phase-locking to harmonics of centimeter-wave radiation from a stabilized microwave synthesizer. The phase-locked submillimeter-wave radiation has the frequency stability of a couple of kHz and the output power on the order of milliwatts. An InSb hot electron bolometer cooled with liquid helium was used for the detection of the submillimeter-wave radiation. A double modulation scheme was used as described in Amano & Maeda (2000); Amano & Hirao (2005).

The molecular constants and the predicted rotational transition frequencies given by Rösslein et al. (1991) and Jagod et al. (1992) were a good starting point in searching for the rotational lines. The electric dipole moment is calculated to be 0.329 D, which lies along the a principal inertia axis, resulting in the a-type rotational transitions. Due to equivalent protons, the 3:1 nuclear spin statistics appears to the K_a odd (ortho) and K_a even (para) states.

At first, a very weak feature was found almost exactly at the calculated frequency for the 2₁₂-1₁₁ transition. Eventually the line appeared strong enough for precise frequency measurements after adjusting the reaction conditions. The observations were made with the discharge current of about 16 mA with liquid nitrogen cooling. The optimum gas mixture was found to be CH₄ (3 mTorr), CD₄ (1 mTorr), H₂ (2 mTorr), and He (35 mTorr). Here helium played an important role more than a buffer gas, but was found to be essential to produce CH₂D⁺. No signals were detectable with the Ar buffer. In the helium-dominated plasma, the electron temperature is higher than that in argon dominated plasma, and helium ion and metastable exited helium are abundant. The ionization of CH₄ and CD₄ should be the initial step of the formation processes. Energetically all CH₄⁺, CH₃⁺, CH₂⁺, and CH⁺ can be generated (Crofton et al. 1985). The reactions in the plasma are complicated, and from the submillimeter observation alone we cannot identify specific processes to contribute to the formation of CH₂D⁺. It is interesting that adding D₂ instead of CD₄ resulted in no signal. Although the signal was seen without H₂, it appears to play a subtle role in the formation, resulting in about a factor 2 increase in intensity.

Table 1:   Observed transition frequencies for CH₂D⁺ (in MHz).

As this ion is a light asymmetric molecule and the signal was only weakly observed, four transitions were detected so far in the 280-890 GHz region. Figure 1 shows a typical example of the observed signals. All observed transition frequencies agree within 1 MHz of the predicted frequencies given by Rösslein et al. (1991), as listed in Table 1. The energy level diagram for the low-J states is illustrated in Fig. 2. Two astronomical lines identified as due to CH₂D⁺ by Lis et al. (2009) are consistent with the laboratory frequencies.

The accuracy of the submillimeter-wave lines is on the order of a few tens of kHz, while the accuracy of the rotational transition frequencies calculated with the molecular constants derived from the infrared measurements can be as high as about 30 MHz. So, although the frequencies measured in this work agree extremely well with the predicted frequencies from the infrared data by Rösslein et al. (1991), these submillimeter-wave lines should be very useful to obtain improved predictions of other frequencies by a combined reanalysis of the combination differences from the infrared bands and the submillimeter-wave lines. Thus a combined analysis was carried out by using the combination differences derived from the transition wavenumbers for both the and fundamental bands given in Jagod et al. (1992). We took 307 combination differences that involved the energy levels of less than J = 10, from which we excluded 83 combination differences from the fit. The excluded combination differences were obviously not as accurate, showing more than 0.005 cm^-1 deviations. The standard deviation of the fit for the infrared combination differences was found to be about 45 MHz. Table 2 lists the molecular constants determined from the least-squares fit together with those obtained by Rösslein et al. (1991) and Jagod et al. (1992) for comparison. The predicted rotational transition frequencies between the levels located below 4 THz (see Fig. 2) are shown in Table 3.

Table 2:   Molecular constants for CH₂D⁺ in the ground state (in MHz).

Table 3:   Predicted rotational transition frequencies between the low-lying rotational levels of CH₂D⁺ in the ground state (in MHz).

3 Discussion

The signals exhibited typical characteristics of positive ions in extended negative glow discharges; i.e. the lines were only observable with a magnetic field over approximately 100 Gauss, and the intensity decreased dramatically by warming the cell wall from liquid nitrogen temperature. All experimental evidence supports the identification of the species detected in this experiment to be CH₂D⁺.

Various efforts to improve the signal intensity were unsuccessful, and only one para line was observed. The lowest para transition, 1₀₁-0₀₀, at 278 691.7 MHz is located at the lowest frequency edge covered by our BWO oscillator. In the helium discharge, plasma interaction tends to be more severe, especially in the lower frequency region, so the severe baseline distortion hampers the detection of this line even with the double modulation scheme with the on-off magnetic field modulation. Two other lines, 2₀₂-1₀₁ at 534 280 MHz and 3₀₃-2₀₂ at 757 259 MHz, should have a similar intensity as the 3₂₂-2₂₁ line at 100 K. However, these frequencies fall into the frequency gaps of the spectrometer by a very unfortunate coincidence, and cannot be measured with the current setup.

As shown in Tables 2 and 3, an inclusion of the submillimeter-wave lines, albeit only four lines, results in a substantial improvement in determination of the molecular constants and the rotational line frequencies. The notable improvement of the accuracy is evident for B and C. Although the accuracy for the A rotational constant is significantly improved,the degree of the improvement is not as dramatic as for B and C, because the lines observed here do not strongly depend on A. In the fit, other higher-order centrifugal distortion constants were not significantly determined. These parameters and the predictions should prove to be useful in further astronomical searches and laboratory investigations. The predicted frequencies listed in the Cologne database (CDMS) agree mostly with the values in Table 3 within their given uncertainties.

4 Conclusions

Four submillimeter-wave lines of CH₂D⁺ have been definitely identified, and the rest frequencies are obtained to uncertainties of 20-50 kHz. These rest frequencies should be instrumental in the identification of interstellar CH₂D⁺. A combined least-squares fit with the submillimeter-wave lines and the combination differences yielded significantly improved molecular constants and the predictions of the rotational line frequencies related to the relatively low-lying energy levels.

Acknowledgements

The financial support from Natural Science and Engineering Research Council of Canada (NSERC) is gratefully acknowledged. This research was made possible also by the support from the University of Waterloo. I thank Professor Roueff for communicating to me astronomical detections, and Professor Oka for sending me the original infrared data. I am also grateful to Oliver Poon and Wei-Jo Ting for their help at the earliest stage of the experiment.

References

Footnotes

... about 45 MHz

A complete list of the fitted results is available from the author.

... (CDMS)

http://www.astro.uni-koeln.de/cdms/

All Tables

Table 1:   Observed transition frequencies for CH₂D⁺ (in MHz).

Table 2:   Molecular constants for CH₂D⁺ in the ground state (in MHz).

Table 3:   Predicted rotational transition frequencies between the low-lying rotational levels of CH₂D⁺ in the ground state (in MHz).

All Figures

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