High-Resolution Near-Infrared Spectroscopy of CH2+ and Its

Article pubs.acs.org/JPCA

High-Resolution Near-Infrared Spectroscopy of CH2+ and Its Deuterated Isotopologues Haiming Wang,*,†,§ Christopher F. Neese,*,†,⊥ Christopher P. Morong,† Maria Kleshcheva,† and Takeshi Oka†,‡ †

Department of Chemistry and the Enrico Fermi Institute and ‡Department of Astronomy & Astrophysics, University of Chicago, Chicago, Illinois 60637, United States S Supporting Information *

ABSTRACT: Aiming to provide approximate rotational constants for millimeter wave spectroscopists to identify the corresponding species in space, we have recorded the near-infrared spectra of the methylene cation CH2+ and its deuterated isotopologues, CD2+ and CHD+, using a high resolution and high sensitivity spectrometer. Detection of CH2+ in space will shed light on interstellar chemistry as it is the intermediate between the abundant CH+ and yet to be observed CH3+, which is important in the formation of larger organic molecules. CH2+ and its deuterated isotopologues are also of special interest for theoretical studies because of their unique intramolecular dynamics, i.e., the Renner−Teller interaction and quasi-linearity. This paper will discuss several new bands of CH2+, the Ã (0,5[11],0)0 ← X̃ (0,0[0],0)1 and Ã (0,4[11],0)2 ← X̃ (0,0[0],0)1 bands of CD2+, which have been identified and analyzed, and the candidate lines for the Ã (0,4[10],0)1 ← X̃ (0,0[1],0)0 band of CHD+, in comparison with the theoretical predictions by Bunker and colleagues.

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rotational spectrum. The detection of CH3+ in interstellar space via its infrared spectrum has not been successful so far.13 An inconclusive observation of CH2D+ through its rotational spectrum has been mentioned.14 Sandwiched by the spectroscopically simple but chemically enigmatic CH+, whose existence in the diffuse interstellar medium has been known for over 70 years, and the abundant but hard to detect CH3+, CH2+ is clearly a very important species. Observation of this species in interstellar space will increase our understanding of the chemistry of CH+. Obtaining the laboratory spectra of CH2+ and its deuterated isotopologues has been one of the primary goals in our group since recording the laboratory spectra of CH3+ and its deuterated isotopologues.15,16 There is also considerable theoretical interest in CH2+. The ground electronic state of CH2+ in a linear configuration is a 2Πu configuration that splits into two states, X̃ 2A1 and Ã 2B1, as the molecule bends due to the Renner− Teller effect. Because of this effect, there have been extensive theoretical calculations on CH2+.17−24 The X̃ state is quasilinear, with a HCH band angle of approximately 140° and a barrier to linearity (from the minimum of the X̃ state potential) of about 1000 cm−1.

INTRODUCTION After H3+, which plays the central role in interstellar chemistry,1 molecular ions containing one heavy atom, such as the cations OH+, H2O+, H3O+; NH+, NH2+, NH3+, NH4+; and CH+, CH2+, CH3+, CH4+, CH5+ and the anions OH−, NH−, and NH2−, are important reaction intermediates both in the laboratory and in space.2 Many of them have been studied by infrared spectroscopy systematically at the Oka Ion Factory (see Section 3.1 of ref 2 and Section 9 of ref 3). Those spectroscopic studies in this and other laboratories have provided a basis for the recent avalanche of Herschel HIFI discoveries of OH+, H2O+, and H3O+, which opened up a new vista of oxygen chemistry and the determination of the ionization rate in interstellar space.4−12 Carbocations are important molecular ions leading to the production of many varieties of organic compounds that are observed abundantly in interstellar space; CH3+, in particular, is central in interstellar chemistry second only to H3+. The chain of hydrogen abstraction reactions CH+ + H 2 → CH 2+ + H

CH 2+ + H 2 → CH3+ + H

proceeds very efficiently just like OH+ + H 2 → H 2O+ + H

Special Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry

H 2O+ + H 2 → H3O+ + H

Received: December 31, 2012 Revised: July 16, 2013 Published: August 19, 2013

but the production of CH+ is not well understood and planar CH3+, unlike pyramidal H3O+, is not polar and does not have a © 2013 American Chemical Society

9908

dx.doi.org/10.1021/jp3128803 | J. Phys. Chem. A 2013, 117, 9908−9918

The Journal of Physical Chemistry A

Article

Figure 1. Comparison of near- and mid-IR spectra of CH2+. The absence of the dense spectra of CH3+, C2H2+, and C2H3+ greatly facilitates the assignment of the irregular spectrum of CH2+.

achieved because we could not obtain the ΔKa = 1 ground state combination difference due to the selection rules of the vibronic transitions. The laboratory prediction of pure rotational transitions therefore has uncertainty on the order of a few cm−1. We also report first observations of the spectra of CD2+ and CHD+. They confirm and will help improve theoretical calculations of the Renner−Teller interaction by the group of Bunker and Jensen.19−23 They also are relevant to astrochemistry. Although deuterium has been decreasing ever since the Big Bang nucleosynthesis and has very small abundance, the energy difference between zero-point vibrations of the C−D and C−H stretching and the DCD (or DCH) and HCH bending vibration combined with the low temperature of the environment may cause huge isotopic fractionation. Usually the huge deuterium fractionation is explained as due to a deuterium abstraction reaction with HD,

The first spectrum of CH2+ was obtained in 1991 as the Ka = 0 series of the antisymmetric C−H stretching v3 band.25 The Ka = 0 series lines were regular and gave accurate (B + C)/2 rotational constants for the ground and the v3 vibrational states. When we proceeded to the Ka = 1 series, however, we noted that the spectral lines were irregular and hard to assign, so much so that we needed to use chemical discrimination to tell CH2+ lines from stronger CH3+ lines. This irregularity for Ka ≠ 0 series is due to the Renner−Teller interaction, which mixes the ground state with the excited Ã electronic state. After a year, the Ka = 1 laboratory lines were provided to the theoretical group of Bunker and Jensen who were conducting ab initio calculations on CH2+ especially on the Renner−Teller interaction. These lines were published as a part of their extensive theoretical calculations on CH2+, CHD+, and CD2+.20 Those experiences prompted us to move from the midinfrared spectroscopy of the 3 μm C−H stretch to near-infrared spectroscopy of the Renner−Teller split Ã 2 B1 ← X̃ 2A 1 electronic transition. A high-resolution and high-sensitivity Ti:sapphire laser based near-infrared spectrometer using a heterodyne detection system was constructed for the double purpose of observing near-infrared H3 + overtone and combination bands and the Renner−Teller transition of CH2+. This led to the first observation of the electronic spectrum of CH2+. Three vibronic bands, Ã (0,3[8],0)1 ← X̃ (0,0[1],0)0, Ã (0,3[9],0)2 ← X̃ (0,0[0],0)1, and Ã (0,4[9],0)0 ← X̃ (0,0[0],0)1 have been analyzed and led to the ground state rotational constants B and C.26 In the meantime, the zerokinetic energy (ZEKE) photoelectron spectroscopy conducted in Merkt’s laboratory27,28 determined the sorely needed A rotational constant in the ground state to be 67.65(81) or 69.53(68) cm−1 depending on the fitting method, leading to an experimentally determined H−C−H angle of 139.8°. In this paper, we report observations of four new vibronic bands of CH2+, which were either outside of the previous wavelength region or below the sensitivity limit. We also finalize some previously tentative assignments of three bands with minor line reassignments. Our original goal to provide accurate predictions of the pure rotational 111 ← 000 transition of paraCH2+ and 110 ← 101 transition of ortho-CH2+ has not been

H3+ + HD → H 2D+ + H 2

H 2D+ + HD → HD2+ + H 2

HD2+ + HD → D3+ + H 2

with exothermicities on the order of 200 K.29 Parise et al. pointed out that the deuterium abstraction reaction of carbocations with HD, CH3+ + HD → CH 2D+ + H 2

CH 2D+ + HD → CHD2+ + H 2

CHD2+ + HD → CD3+ + H 2

will work efficiently at higher temperatures because their exothermicities are much higher than those of the H3+ chain.30 This does not work for CH2+ because it will be destroyed by HD but the deuteron−proton exchange reaction with D, CH 2+ + D → CHD+ + H CHD+ + D → CD2+ + H 9909



Article

Table 1. Observed Lines of the Ã (0,3[8],0)1 ← X̃ (0,0[1],0)0 Band of CH2+ line

vF1/cm−1

vF2/cm−1

112−202 213−303 314−404 415−505 516−606 617−707 718−808 819−909 91,10−100,10 101,11−110,11

11110.5862 11094.9417 11078.7898 11062.6409 11046.2232 11029.5868 11012.8521 10995.8971 10981.2944 10965.0681

11111.6058 11095.4420 11080.0714 11063.3853 11046.7332 11030.0106 11013.3219 10996.5404 10981.7347

111−101 212−202 313−303 414−404 515−505 616−606 717−707 818−808 919−909 101,10−100,10 111,11−110,11

11139.0750 11136.0889 11131.4881 11125.3816 11117.8784 11109.1127 11099.2435 11088.4702 11077.0328 11065.2471 11053.4111

11140.1654 11136.6976 11131.8918 11125.6688 11118.0911 11109.2679 11099.3579 11088.5634 11077.0909 11064.8783

110−000 211−101 312−202 413−303 514−404 615−505 716−606 817−707 918−808 1019−909

11154.8570 F21: 11155.8882 11168.6817 11181.9268 11195.0827 11207.8525 11220.2553 11232.3970 11244.1430 11258.0264 11270.1079

by Gottfried et al.31 and Morong et al.32 A combination of frequency modulation via an electro-optic modulator operated at 500 MHz with heterodyne detection and velocity modulation via the 19 kHz ac discharge with bidirectional optical multipassing yielded a second derivative Gaussian line shape that allowed for spectroscopy with near shot-noise-limited sensitivity. The ac discharge velocity modulates only the ions, while keeping the neutrals unaffected. By comparing the spectra of CD4, CH4, and a 1:1 mixture of both under the same plasma conditions, one can identify the lines of CH2+, CD2+, and CHD+ against those of interfering ionic impurities, which will be discussed in detail in the following section. Most of the region was scanned at 5 cm−1/h to obtain sufficient S/N for a conclusive identification.

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THEORETICAL METHODS

Rotational Labels. The rovibronic structure of CH2+ is best described in a Hund’s case (b) basis. The quantum numbers that describe the Hund’s case (b) basis are J, the total angular momentum excluding nuclear spin; N, the rotational angular momentum; S, the electronic spin angular momentum; MJ, the projection of J on a laboratory-fixed axis; and Ka, the projection of N on the molecule’s axis of linearity. None of these quantum numbers are rigorous, but for CH2+ they are all approximately good quantum numbers. CH2+ has one unpaired electron (S = 1/2) making each transition a doublet. The two spin components are labeled F1 (J = N + 1/2) and F2 (J = N − 1/2) with the approximate selection rules being F1 ↔ F1 and F2 ↔ F2. F1 ↔ F2 satellite transitions are weaker but significant at low J values. These lines are labeled using a shorthand, where F1, F2, F12, and F21 stand for F1 ← F1, F2 ← F2, F1 ← F2, and F2 ← F1, respectively. The rotational levels are labeled NKaKc, using the standard spectroscopic notation for asymmetric tops. Ka is the value of |K| in the prolate limit and Kc is the value of |K| in the oblate limit. The parity of the rotational wave function is positive when Kc is even and negative when Kc is odd.33 The parity selection rule + ↔ − is rigorous. For CH2+ and CD2+, the symmetry of the rotational wave function with respect to permutation of the equivalent H or D nuclei is given by the parity of Ka + Kc. Spin statistical weights are manifested in the spectrum as intensity alterations. In the ground vibronic state, Ka + Kc = odd corresponds to orthoCH2+ and para-CD2+, whereas Ka + Kc = even corresponds to para-CH2+ and ortho-CD2+. The selection rule ortho ↔ ortho and para ↔ para is rigorous for these isotopologues. CH2+ has a 3:1 ortho:para intensity ratio, whereas CD2+ has a 2:1 ortho:para intensity ratio. Vibronic Labels. There are two systems in place to label the vibronic terms of a Renner−Teller molecule. Both systems are based on a normal-mode analysis of the three vibrational modes of a triatomic molecule. The first notation is a “linear notation” corresponding to treating the molecule as an unsplit linear 2Π state. This is the notation used by Pople and Longuet− Higgins in the first analysis of the Renner−Teller effect in NH2.34 The second notation is a “bent notation”, corresponding to treating the molecule as having two noninteracting electronic states. The bent notation is used exclusively in the ab initio calculations from the RENNER program. In the linear notation, each vibronic state is labeled by the numbers v1, vlin 2 , and v3 and Ka = |l ± Λ|. In this representation, Ka may be considered a vibronic quantum number, the sum of

11169.1963 11183.2231 11195.8426 11208.3711 11220.6924 11232.8860 11244.8004 11258.4844 11270.3031

may work under some conditions because their exothermicities are much larger.

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EXPERIMENTAL DETAILS The molecular ions were produced using a positive column discharge in a triple jacketed glass tube. The innermost tube contained the plasma and the outermost jacket was held under vacuum for thermal insulation. The middle jacket contained liquid nitrogen to maintain the rotational temperatures of the produced ions around 300 K. A mixture of a 100 mTorr reagent gas (CH4, CD4, or CH4 + CD4), plus a 10 Torr He buffer gas was ionized by applying a 200 mA (rms) alternating current at 19 kHz. The CH 2+ /CD2 +/CHD+ ions were produced effectively via Penning ionization in the plasma cell. It was found that the strength of CH2+ does not vary significantly with methane pressures between 10 and 100 mTorr. In the mid-IR, we found that it was optimal to operate with 10 mTorr of methane to minimize the strength of interfering CH3+, C2H2+, and C2H3+ lines. In the near-IR, none of these carboions have strong spectra, and it was optimal to operate at 100 mTorr of methane to minimize the strength of interfering N2+ lines. Spectra were recorded using a near-infrared spectrometer based on a Ti:sapphire laser, which has been explained in detail 9910



Article


vF1/cm−1

vF2/cm−1

line

vF1/cm−1

vF2/cm−1

110−220

10835.4867

10839.8460

111−221

10834.4669

211−321

10819.8651

212−322

10816.8140

312−422 413−523 514−624 615−725 716−826 817−927 918−1028 1019−1129

10803.4383 107867375 10769.4740 10751.6337 10733.2919 10714.2391

10822.6419 F12: 10822.1200 10806.4647 10788.9005 10771.1788 10753.0972 10734.6829 10715.7131 10698.1875 10678.3909

10838.8923 F12: 10837.7950 10819.6892

313−423 414−524 515−625 616−726 717−827 818−928 919−1029

10797.3221 10776.1721 10753.4801 10729.3742 10704.0069 10677.5304 10650.2189

10799.4631 10777.8650 10754.8868 10730.5673 10705.0374 10678.4646 10651.0200

211−221 312−322 413−423 514−524 615−625 716−726 817−827 918−928 1019−1029

10864.0718 10862.6452 10860.9185 10858.6435 10855.8562 10852.6716 10848.9032 10847.1037 10843.2939

10867.9219 10866.2095 10863.4147 10860.5749 10857.4866 10854.1829 10850.4782 10848.3908 10844.2314

311−221 412−322 513−423 614−524 715−625 816−726 917−827

10900.6199 10909.2428 10916.1564 10921.5304 10925.5105 10928.2827 10930.0404

10904.3667 10911.8136 10918.1139 10923.1088 10926.8330 10929.4312 10931.0306

1119−1029

10931.6397

210−220 311−321 412−422 513−523 614−624 715−725 816−826 917−927 1018−1028

411−321 512−422 613−523 714−624 815−725 916−826 1017−927

10677.5315

10821.2921 10808.9039 10795.3778 10780.9300

10864.9263 10859.0874 10852.0617 10843.6076 10833.7145 10822.4332 10809.9193 10796.2582 10781.3236

10920.0068 10932.4962 10944.3527 10955.6479 10966.1823 10978.4607 10988.4592

10923.0363 10934.7737 10946.2125 10957.3321 10967.8857 10979.8419 10989.4744

10856.4093 10850.0300 10841.9792

Figure 2. Stick spectrum of CH2+, predicted from ab initio calculations. Newly assigned lines are red, previously assigned lines are blue. The number in brackets is vlin 2 for the upper state.

the electronic angular momentum Λ, and the vibrational angular momentum l. For the X̃ and Ã states of CH2+, Λ = 1. In the bent notation, each vibronic state is labeled by the numbers v1, vbent 2 , and v3. In this representation, Ka is usually

considered a rotational quantum number. The two stretching vibrational quantum numbers v1 and v3 are naturally the same in both representations. The different bending quantum numbers are related by 9911



Article


vF1/cm−1

433−523

10614.9510

635−725

10575.4368

331−321 432−422 533−523 634−624 735−725 836−826 937−927 1038−1028 1139−1129

10692.3887 10689.0443 10684.6432 10676.6326 10671.0109 10662.9754 10653.3576 10642.3438 10629.8627

vF2/cm−1

line

v2lin = 2v2bent + |Ka ± Λ|

10748.2196 10758.6542 10768.1505 10777.8653 10781.0484 10787.1898 10791.9523

10633.3085 10615.1769 10596.1455

10637.9401

737−827

10553.7294

10555.7991

332−322 433−423 534−524 635−625 736−726 837−827 938−928 1039−1029

10692.4073 10689.1389 10684.7937 10679.6542 10674.8695 10663.7647 10655.8302 10646.7887

10693.0457 10687.9098 10682.1281 10676.5399 10665.5632 10657.4176 10648.1171

10598.9965

10577.7352

10697.5557 10692.9559 10687.7189 10678.5817 10673.1966 10664.7753 10654.9333 10643.6581 10630.9414

432−322 533−423 634−524 735−625 836−726 937−827 1038−928 1139−1029

10752.6794 10762.0950 10770.8482 10779.6875 10782.9698 10793.3571

10736.6045 F21: 10739.5020 10748.2409 10758.8230 10765.7982 10775.2385 10782.3490 10788.0210 10792.4644

10742.8362 10752.7135 10762.2274 10767.9820 10777.5893 10784.2773 10789.6971 10793.8644 10796.7770

Table 4. Observed Lines of the Ã (0,4[9],0)0 ← X̃ (0,0[0],0)1 Band of CH2+

(1)

Either the bent or linear notation can be used to uniquely label each vibronic term. However, neither notation alone does a very good job in describing the vibronic structure of CH2+. The reason for this is that below the barrier to linearity, vbent is 2 the better quantum number, but above the barrier to linearity vlin 2 is better. Therefore, we have chosen to label terms using the notation: ̃ v1 , v2bent[v2lin], v3)Ka X̃ |A(

vF2/cm−1

333−423 434−524 535−625

331−221 431−321 532−422 633−523 734−624 835−725 936−826 1037−927

vF1/cm−1

(2)

Occasionally, when referring to multiple vibronic states, Ka and lin either vbent or vlin 2 2 are omitted. Values of v2 are always reported in square brackets. We could have chosen simply to use bent notation for the X̃ and linear notation for the Ã state. We have decided against this for several reasons. First, having inconsistent notation for the two states is potentially confusing, especially because they comprise a Renner−Teller pair of states. Second, the ab initio calculations use the bent notation exclusively; the mixed notation is readily compatible with this. Third, although we are primarily interested in the ground vibrational state of the X̃ state, which is best described in bent notation, the vast majority of X̃ state vibrational levels are better described using the linear notation, as the barrier to linearity is quite low. Finally, vbent always can be interpreted as the node count of the 2 radial bending coordinate ρ, even when the molecule is linear.

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RESULTS AND DISCUSSION CH2+. As mentioned in the Introduction, in the mid-IR the irregular spectrum of CH2+ is obfuscated by other carbocations, especially CH3+, C2H2+, and C2H3+. This led us to shift our attention from the mid-IR spectroscopy of the 3 μm C−H

line

vF1/cm−1

vF2/cm−1

000−110

12241.8285

101−211 202−312 303−413 404−514 505−615 606−716 808−918

12225.4359 12207.4634 12188.1044 12167.5430 12145.9970 12123.7383 12078.1998

F12: 12244.3711 12226.7475 12208.3724 12188.8040 12168.1229 12146.5080 12124.2072 12078.6519

101−111 202−212 303−313 404−414 505−515 606−616 707−717 808−818

12256.3066 12255.2495 12253.4975 12251.2703 12248.7790 12246.2732 12243.9794 12242.2251

12258.8483 12256.5491 12254.3821 12251.9472 12249.3265 12246.7537 12244.4354 12242.6428

202−110 303−211 404−312 505−413 606−514 707−615 808−716

12282.8637 12293.9454 12303.6460 12312.2299 12319.9741 12327.1152 12334.0116

12285.4013 12295.2598 12304.5691 12312.9614 12320.5966 12327.7035 12334.5466

stretch to near-infrared spectroscopy of the Renner−Teller split Ã 2B1(Πu) ← X̃ 2A1 electronic transition. With the spectrum of 9912



Article


vF1/cm−1

vF2/cm−1

vF1/cm−1

line 220−312

322−414

12090.1799

12094.2301

524−616

12058.1553

12060.6736

vF2/cm−1

12100.3810

12106.0241

422−514

726−818 220−212

12064.9193

624−716

12016.5054

12026.2028 12154.1951 F12: 12149.4666 12151.5473

221−211

12145.7526

12151.7994

321−313

12148.1702 F21: 12152.8955 12147.2821

322−312

12142.5562

422−414 523−515 624−616 725−717 826−818

12145.5082 12142.8379 12139.0396 12134.0994 12127.8412

12148.7450 12145.3564 12141.1133 12135.8020 12129.2598

423−413 524−514 625−615 726−716

12137.8777 12131.8560 12124.6032 12116.3560

12146.8574 F12: 12143.4772 12141.1713 12134.5061 12126.7914 12118.1041

928−918

12096.4384

12097.3161 12183.0527

12201.3684 12208.9806 12214.9502 12219.0675 12221.1708

221−111

12183.9015

220−110

12195.0318

321−211

12175.7825 F21: 12180.5037 12187.7264

423−313 524−414 625−515 726−616

12176.6257 F21: 12181.3529 12190.3427 F21: 12193.7310 12203.2692 12215.5823 12227.3839 12238.8922

12206.7512 12218.3294 12229.6225 12240.6554

422−312 523−413 624−514 725−615 826−716

12197.8864 12206.2949 12212.7376 12217.2339 12219.6296

928−818

12260.4626

12261.3106

322−212

12192.4334


vF1/cm−1

vF2/cm−1

322−432 423−533 524−634 625−735 726−836

11572.7725 11554.2839 11534.6474 11513.9988 11492.6177

11578.9190 11559.1025 11538.5911 11517.3199 11495.4497

11622.6858 11615.5319 11606.6361 11595.8980

11633.4302 11626.9031 11619.0418 11609.5988 11598.5225

422−432 523−533 624−634 725-735 826−836 524−432

line

vF1/cm−1

vF2/cm−1

220−330 321−431 422−532 523−633 624−734

11589.9521 11572.6064 11553.7764 11533.4704 11511.5137

11598.3238 11578.7443 11558.5725 11537.3362 11514.7572

322−330 423−431 524−532 625−633 726−734

11632.1259 11628.6000 11623.8497 11618.0230 11611.3597

11639.1545 11633.9569 11621.5983 11614.3004

11703.0192 624−532

11704.7421

11708.6099

Ã (0,3[8],0) ← X̃ (0,0[1],0) and Ã (0,2[8],0) ← X̃ (0,0[1],0)2 bands, which would provide the first experimental lines from Ka = 2 in the ground state. These bands were below the frequency region of Gottfried et al., as they required the longwave optics set for the Ti:sapphire laser. Using this optics set, we scanned the full tuning band, 10628−11325 cm−1, which also included the previously studied Ã (0,3[8],0)1 ← X̃ (0,0[1],0)0 band. The most prominent feature was the R-branch of the Ã (0,2[8],0)3 ← X̃ (0,0[1],0)2 band, as the K-type doubling of this band is unresolved at low N. This band was also easier to assign, as the Ka > 1 lines are more regular than the Ka = 1 lines. Furthermore, once the K-type doubling

Gottfried et al., we initially hoped that more lines of the 3 μm IR spectrum might be assigned. To this effect, we recorded a new 3 μm IR spectrum with a double-modulation spectrometer similar to the NIR spectrometer using a Burleigh FCL-20 color center laser. In Figure 1 the IR spectrum is compared with the NIR spectrum described below. The 16 cm−1 regions displayed were selected to show R(2) lines. In the region shown, the NIR spectrum shows two CH2+ lines and a weak H2-Rydberg line (3p ← 2s(2,1)P(2) at 10732.82 cm−1.35). In contrast, the 3 μm spectrum contains over a hundred lines. It quickly became apparent that it would be more efficient to expand our NIR spectroscopy. We began by searching for the

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Article


vF1/cm−1

vF2/cm−1

744−634

11715.3849

946−836

11728.5218

440−432

11393.6791

11402.7804

642−634

11384.4626

11390.6838

844−836

11370.6784

542−432

11464.2212

11471.9463

744−634

11482.0699

11487.6364

line

vF1/cm−1

vF2/cm−1

844−734

11714.6399

11717.3614

542−532

11389.8995

11397.0903

744−734

11378.0542

440−330

11453.0303

11463.0136

642−532

11473.6633

11480.2505

844−734

11489.4140

11494.2094

Figure 3. Spectrum of CD2+, predicted from ab initio calculations. Lines with intensities below 104cm/mol are excluded for clarity.

recording a ∼1 cm−1 region around these lines with an improved signal-to-noise ratio provided by coadding multiple scans. The final assignment of these three bands is presented in Tables 1−3. The assignments are unambiguous without spectral modeling because most energy levels are connected by 4−6 spectral lines. After assigning the above bands, we decided to extend our data set to Ka = 3 in the X̃ state by assigning the Ã (0,3[9],0)2 ← X̃ (0,0[2],0)3 and Ã (0,2[9],0)4 ← X̃ (0,0[2],0)3 bands. These bands were mostly below the sensitivity limits of Gottfried’s original survey. Using the midwave optics set of the Ti:sapphire laser, we scanned the 11320−12385 cm−1 region, which included the previously assigned Ã (0,4[9],0)0 ← X̃ (0,0[0],0)1 and Ã (0,3[9],0)2 ← X̃ (0,0[0],0)1 bands. The assignment of these two new bands was rapid, as the two previously assigned bands were already self-consistent as they

resolves at higher N, the 3:1 nuclear spin intensity alteration clearly identifies the correct lines. The Q-branch of this band was also fairly easy to identify, although it is slightly weaker. The P-branch was below the tuning range of the laser. With tentative R − Q ground state combination differences from the Ã (0,2[8],0)3 ← X̃ (0,0[1],0)2 band, we were able to search for lines in the more irregular Ã (0,3[8],0)1 ← X̃ (0,0[1],0)2 band. Correct identification of a R−Q pair of lines immediately predicted out the P line, as we already had the Ã (0,3[8],0)1 ← X̃ (0,0[1],0)0 band tentatively assigned. The only catch is that we had to finalize the assignment of the Ã (0,3[8],0)1 ← X̃ (0,0[1],0)0 band, which required a minor reassignment of the electronic spin-splitting. With this correction, the assignment proceeded quickly. We confirmed the presence of weak lines critical to the assignment (especially ΔJ ≠ ΔN satellite lines) by 9914



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Figure 4. Discharge spectra of 10 Torr He and 0.1 Torr (a) CD4 and (b) CH4. The colored lines are the actual data; the black curves are the best fit to a second derivative Gaussian line shape. By examining both spectra, we could identify CD2+ lines. Lines common to both spectra are most likely C2+.

Table 8. Observed Lines of the Ã (0,5[11],0)0 ← X̃ (0,0[0],0)1 Band of CD2+ line

vF1/cm−1

vF2/cm−1

000−110 101−211 202−312 303−413 404−514 505−615 606−716 707−817

11249.1718 11240.9638 11232.0292 11222.4711 11212.4496 11202.1106 11191.6089

11250.4513 11241.6215 11232.4678 11222.8004 11212.6935 11202.2763 11191.6928 11181.1054

101−111 202−212 303−313 404−414 505−515 606−616

11256.4624 11255.9383 11255.1133 11254.1472 11253.1905 11252.3921

11257.7408 11256.5873 11255.5423 11254.4419 11253.3830 11252.4964

303−211 404−312 505−413 606−514 707−615

11275.8244 11281.0792 11285.9841 11290.6925 11295.3500

11276.4636 11281.4961 11286.2595 11290.8436 11295.3799

along with coloring indicating the portions which have been observed to date is shown in Figure 2. The preliminary data from Tables 1−7 were provided to the theoretical group of Bunker and Jensen. Using our experimental data, they were able to calculate small empirical adjustments for the ab initio potential energy surfaces to better model the known experimental lines of CH2+.23 With these corrections, the largest residual was about 4 cm−1 excluding the heavily mixed Ã (0,3[8],0)1 state. CD2+. A predicted spectrum of CD2+ calculated using the empirically adjusted potential from ref 23 is shown in Figure 3.36 On the basis of these predictions, we targeted our search for CD2+ to two regions. The first region, 11140−11370 cm−1, contains the overlapped Ã (0,5[11],0)0 ← X̃ (0,0[0],0)1 and Ã (0,4[11],0)2 ← X̃ (0,0[0],0)1 bands. The second region, 12000− 12400 cm−1, contains the Ã (0,5[12],0)1 ← X̃ (0,0[1],0)0 band. The assignment of the CD2+ lines was more challenging than with CH2+. The only previous experimental spectroscopic work on CD2+ was a PFI-ZEKE spectrum by Willitsch and Merkt in 2003.28 However, due to the congestion from CD2+ transitions, no unambiguous assignment could be made, and no rotational constants were given. Furthermore, the spectrum of CD2+ is about a factor of 2 weaker than the spectrum of CH2+. In the spectrum of a CH4/He discharge, we observed some RydbergH2 lines,35 the Phillips band of C2,37 and a few atomic lines.38 Other than these previously assigned lines, the lines of CH2+ were the strongest lines observed. This was not the case for CD2+ in a CD4/He discharge; therefore, it was necessary to use chemical discrimination to help identify the lines of CD2+ from the lines of other ions. This was done by comparing the spectrum recorded with CD4 with the spectrum recorded using CH4. A typical section of the CD4 spectrum (upper panel) compared with that of CH4 (lower panel) is shown in Figure 4. Many lines were observed in both spectra. The velocity modulated widths of the lines were narrower, indicating that the ion contained two “heavy” atoms, and because the lines existed in both, the ion did not contain H or D. The lines did not match up with N2+ transitions, so the only logical conclusion was the formation of C2+. However, it was clear that the lines were not due to the B̃ 4Σ−u ← X̃ 4Σ+g band of C2+

share a common ground state. Thus, we were able to use combination differences for the Ã (0,3[9],0)2 vibronic state calculated from the Ã (0,3[9],0)2 ← X̃ (0,0[0],0)1 band to assign the Ã (0,3[9],0)2 ← X̃ (0,0[2],0)3 band. We were then able to use combination differences for the X̃ (0,0[2],0)3 state from the Ã (0,3[9],0)2 ← X̃ (0,0[2],0)3 band to assign the Ã (0,2[9],0)4 ← X̃ (0,0[2],0)3 band. The final assignment of these four bands is presented in Tables 4−7. Again the assignments are unambiguous without spectral modeling because most energy levels are connected by 4−6 spectral lines. The assignments for the previously studied Ã (0,4[9],0)0 ← X̃ (0,0[0],0)1 and Ã (0,3[9],0)2 ← X̃ (0,0[0],0)1 bands include seven newly assigned satellite lines. The K-type doubling in the Ã (0,2[9],0)4 ← X̃ (0,0[2],0)3 band is completely unresolved. The complete predicted ab initio spectrum 9915



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Table 9. Observed Lines of the Ã (0,4[11],0)2 ← X̃ (0,0[0],0)1 Band of CD2+ line

vF1/cm−1

vF2/cm−1

line

vF1/cm−1

vF2/cm−1

221−211 322−312 423−413 524−514 625−615 726−716

11237.6256 11235.5912 11233.1074 11234.1615 11226.9834

11235.7242 11234.3849

220−212 321−313 422−414 523−515 624−616 725−717

11238.6554 11237.5329 11233.1074

11236.7522

11231.7202 11228.8373

11235.0492 11233.3388 11231.0431 11228.2194

220−110 321−211 422−312 523−413 624−514

11252.7254 11258.2401 11263.0084

11251.4389 11257.2642 11262.1084 11266.2126

221−111 322−212 423−313 524−414 625−515

11253.1217 11259.5230 11265.7517 11271.9000 11278.0651

11251.8399 11258.5005 11264.7787 11271.0650 11277.2866

11226.1737

Figure 5. Stick spectrum of CHD+, predicted from ab initio calculations. Lines with intensities below 104 cm/mol are excluded for clarity.

Figure 6. Experimental spectra of discharges of methane isotopologues in He. The colored lines are the actual data; the black curves are the best fit to a second derivative Gaussian line shape. A mixture of 10 Torr He and 0.1 Torr (a) CH4, (b) CD4, (c) 1:1 mixture of CH4 and CD4, and (d) CH2D2 was recorded. The spectra of (c) and (d) are similar, showing that similar chemistry exists in both plasmas. Lines common to all four spectra are most likely from C2+.

that we have recently studied with a similar spectrometer in the visible.39 The B̃ ← X̃ bands are violet degraded, whereas the lines in our spectrum are red degraded. Our conclusion was

that these bands must originate from the doublet manifold of C2+. We have subsequently assigned some of these lines and will publish them in a future paper. 9916



Article

The intensities of lines were comparable between the discharge using CH2D2 and the discharge using a mixture of CH4 and CD4, indicating that CHD+ ions were successfully created in both discharges. This unanticipated result resolved the long-standing speculation about the production mechanism of CH2+ ions. It clearly shows that the production of CH2+ (and its deuterated variants) is not a simple one-step Penning dissociative ionization but is a more complicated multistep scrambling of protons. This is further evidenced by the fact that the lines of CH2+ and CD2+ that appear in the CH2D2 discharge have roughly half the intensity as from a discharge using pure CH4 or CD4. A simple one-step Penning ionization would produce CH2+ and CD2+ at a quarter of the intensity when CH2D2 was used as a reagent gas. It was desirable to know that the 1:1 mixture of CH4 and CD4 produces CHD+ with equal efficiency as the more expensive CH2D2. The CHD+ lines found with the above approach are listed in the supplemental table together with lines from other carriers. Although the lines are too sparse to facilitate a complete assignment of the Ã (0,4[10],0)1 ← X̃ (0,0[1],0)0 band, they provide helpful information for further studies of this important ion.

Tables 8 and 9 list the transitions of the overlapped Ã (0,5[11],0)0 ← X̃ (0,0[0],0)1 and Ã (0,4[11],0)2 ← X̃ (0,0[0],0)1 bands. The assignment of these two bands was begun by shifting the predictions from ref 23 until a good set of candidate lines for the Q-branches could be identified. It was found that the Ã (0,5[11],0)0 ← X̃ (0,0[0],0)1 band was predicted about 5 cm−1 too low and the Ã (0,4[11],0)2 ← X̃ (0,0[0],0)1 band was predicted about 7 cm−1 too high. Once the Q-branches of these bands were located, it was possible to assign these bands together, using the criterion that ground state combination differences agree between the two bands. The P-branch of the Ã (0,4[11],0)2 ← X̃ (0,0[0],0)1 is expected to be weak and was not observed. We were only able to assign these two bands up to N = 7 in the excited state. There appears to be a perturbation at N = 8 and we were unable to identify higher N lines in the Q-branch, although there are good candidate lines for higher N in the R-branch. We were unable to assign lines from the Ã (0,5[12],0)1 ← ̃ X(0,0[1],0)0 band. We were somewhat hampered in trying to assign this band without having a second band that would give us combination differences for the X̃ (0,0[1],0)0 state. However we expect combination differences from the X̃ (0,0[1],0)0 state to agree well with the theoretical model. Several candidate CD2+ lines were observed in the region where this band is expected, but we did not find enough lines to begin an assignment. It is quite possible that the Ã (0,5[12],0)1 state is heavily mixed like the Ã (0,3[8],0)1 state of CH2+. CHD+. A predicted spectrum of CHD+ calculated using the empirically adjusted potential from ref 23 is shown in Figure 5.36 This spectrum was calculated at 600 K using an intensity threshold of 10 cm/mol. Compared to the predicted CH2+ and CD2+ spectra, there are noticeably fewer lines predicted in the CHD+ spectrum. This appears to be due to the lines of CHD+ having lower line strengths. Given comparable line strengths, we would expect CHD+ to have intensity intermediate between CH2+ and CD2+. We scanned the 12050−12270 cm−1 region where the vibronic transition Ã (0,4[10],0)1 ← X̃ (0,0[1],0)0 is theoretically predicted and not overcrowded with other bands. To identify the CHD+ lines, we compared spectra from four discharges using different isotopologues of methane as a reagent gas. The four plasma chemistries were CH4, CD4, a 1:1 mixture of CH4 and CD4, and CH2D2 all at a pressure of 100 mTorr in 10 Torr of He. Figure 6 shows a small section of these spectra. Lines that appear in all four discharges are due to C2+, which is not affected by deuteration. Lines that appear in the CH4 discharge and weaker in the CH4 + CD4 and CH2D2 discharges are from CH2+. The same is true for CD2+; lines of CD2+ appear strongly in the CD4 discharge and weaker in the CH4 + CD4 and CH2D2 discharges. The new lines that only appear in the CH4 + CD4 and CH2D2 discharges are those of CHD+. A truth table summarizing identification of line carriers using chemical discrimination is given in Table 10.

■

CONCLUSION We have recorded the near-infrared spectra of CH2+ and its isotopologues with the aim of guiding radio astronomers in identifying this important cation in interstellar clouds. Using chemical discrimination, we have identified many lines that are potentially carried by an isotopologue of CH2+. We have also shown that CH2+ ions are not formed by a simple one-step Penning dissociative ionization but instead are formed by a multistep process that scrambles the protons and deuterons. This means the spectrum of CHD+ can be studied using a mixture of CH4 and CD4 as a reagent gas instead of using the more expensive CH2D2 gas. Seven bands of CH2+ and two bands of CD2+ have been assigned. These assignments are very secure despite the complicated pattern produced by the Renner−Teller effect because the assignments redundantly predict the term energy of each state connected by the observed lines. We are unable to currently calculate the pure-rotational lines from the current set of assigned lines alone. Our assignments should allow further adjustment of the potential energy surface used by the group of Bunker and Jensen to theoretically model the spectrum of CH2+, which will provide a more accurate prediction for the pure rotational spectrum.

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ASSOCIATED CONTENT

S Supporting Information *

Listing of all lines measured in the four plasma chemistries. The carrier of each line is marked based on Table 10. Lines with line centers measured within 0.002 cm−1 of each other are considered to have the same carrier. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Table 10. Chemical Discrimination Truth Table reagent gas

AUTHOR INFORMATION

Corresponding Author

carrier

CH4

CD4

CH2D2 or CH4/CD4

1 .CH2+ 2. CD2+ 3. CHD+ 4. C2+

yes no no yes

no yes no yes

either either yes either

*E-mail: H.W., [email protected]; C.F.N., cfneese@ physics.osu.edu. Present Addresses §

Department of Natural Sciences, Prairie State College, Chicago Heights, IL 60411, USA. 9917



Article

⊥

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Department of Physics, The Ohio State University, Columbus, OH 43210, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the NSF grant PHY03-54200 and NASA grant NMO711043. H.W. also thanks Dr. Masahiro Notani for a handy C program he developed to effectively distinguish the spectral lines.

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REFERENCES

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High-Resolution Near-Infrared Spectroscopy of CH2+ and Its

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