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Lower Rydberg Series of Methane: a Combined Coupled Cluster Linear Response and Molecular Quantum Defect Orbital Calculation A

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Lower Rydberg Series of Methane: a Combined Coupled Cluster Linear Response and Molecular Quantum Defect Orbital Calculation A Lower Rydberg series of methane: A combined coupled cluster linear response and molecular quantum defect orbital calculation A. M. Velasco, J. Pitarch-Ruiz, Alfredo M. Sánchez de Merás, J. Sánchez-Marín, and I. Martin Citation: J. Chem. Phys. 124, 124313 (2006); doi: 10.1063/1.2179069 View online: http://dx.doi.org/10.1063/1.2179069 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v124/i12 Published by the American Institute of Physics. Additional information on J. Chem. Phys. Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors Downloaded 24 Aug 2012 to 152.14.136.96. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions THE JOURNAL OF CHEMICAL PHYSICS 124, 124313 ͑2006͒ Lower Rydberg series of methane: A combined coupled cluster linear response and molecular quantum defect orbital calculation A. M. Velasco Departamento de Química Fisica y Química Inorgánica, Facultad de Ciencias, Universidad de Valladolid, 47005 Valladolid, Spain ͒ J. Pitarch-Ruiz, Alfredo M. J. Sánchez de Merás, and J. Sánchez-Marína Universitat de València, Institut de Ciència Molecular, Edificio de Institutos, Apartado de Correos 22085, E-46071, Valencia, Spain I. Martin Departamento de Química Fisica y Química Inorgánica, Facultad de Ciencias, Universidad de Valladolid, 47005 Valladolid, Spain ͑Received 21 October 2005; accepted 30 January 2006; published online 27 March 2006͒ Vertical excitation energies as well as related absolute photoabsorption oscillator strength data are very scarce in the literature for methane. In this study, we have characterized the three existing series ͑ ͒ of low-lying Rydberg states of CH4 by computing coupled cluster linear response CCLR vertical excitation energies together with oscillator strengths in the molecular-adapted quantum defect orbital formalism from a distorted Cs geometry selected on the basis of outer valence green function calculations. The present work provides a wide range of data of excitation energies and absolute oscillator strengths which correspond to the Rydberg series converging to the three lower ionization potential values of the distorted methane molecule, in energy regions for which experimentally measured data appear to be unavailable. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2179069͔ I. INTRODUCTION genic plasma. The composition of hydrocarbon fluxes enter- ing the plasma covers a wide spectrum of molecules, from The structure and physicochemical behavior of CH has 4 CH to C H .9 attracted the attention of scientists for several good reasons. 4 3 8 Due to its small size and simple electronic structure, Methane is the most abundant of the minor constituents in CH has been the subject of extensive theoretical and experi- the upper atmospheres of the outer planets, and is the domi- 4 mental studies. The accurate determination of its ionization nant continuous photoabsorber in the above atmospheres in the energy region comprised between 8 and 14 eV.1–3 The and fragmentation energies is an important basis for the un- derstanding of many electron-impact phenomena. In particu- observational data confirm that CH4 is present in auroral emissions.4–6 Nevertheless, for a better understanding of the lar, most of the high-energy chemical processes are initiated ͑ ͒ spectroscopic and collisional properties of this molecule at by the interaction of medium-energy electrons 5–50 eV various temperatures and under the influence of different per- with matter. turbers, new experimental and theoretical investigations are Methane is a representative example of a highly sym- clearly needed. metrical molecule affected by the Jahn-Teller effect. If one of Methane is a natural gas emitted from areas such as rice the six 1t2 electrons of CH4 is vertically excited into a Ryd- + cultures, forests, and farms, and also from industrial areas berg state, the resultant CH4 ion core suffers a strong Jahn- and gas pipelines. Despite its natural origin, a growing num- Teller distortion and can be considered to remain in a high ber of sources for this compound are man made nowadays. vibrational state: the tetrahedral structure of neutral methane Thus, such hydrocarbon is increasingly present in the atmo- becomes distorted in the lowest state of methanium cation. sphere, but it usually reacts away very rapidly with OH radi- The photoionization peak of the six electrons in the 1t2 10 cals. As a consequence, its influence on the Earth’s climate is valence orbital is spread out between 12.5 and 16 eV. The much less severe than that of fluorinated hydrocarbons. On center of gravity of this broad peak lies approximately at the other hand, electron collisions with hydrocarbon mol- 14.25 eV, almost 2 eV beyond the adiabatic ionization po- ͑ ͒ ecules are important in many other fields of science and tech- tential IP of CH4. The first band in the photoelectron spec- 7 11 nology, ranging from astrophysics to semiconductor trum of CH4, as recently determined by Kimura et al. with processing.8 For instance, in almost all currently operating photoelectron spectroscopy, is the terminating member of a fusion processes, plasma-wall interactions are an abundant Rydberg series, and displays three maxima, at 13.6, 14.4, and source of hydrocarbon molecules that contaminate the hydro- 15.0 eV, respectively. The vibrational envelope of this band is described as resulting from the Jahn-Teller splitting of the ͒ 12–16 a Author to whom all correspondence should be addressed. Electronic mail: degenerate state of the ion. 10,12,17 [email protected] Other several studies indicate that a distortion of 0021-9606/2006/124͑12͒/124313/8/$23.00124, 124313-1 © 2006 American Institute of Physics Downloaded 24 Aug 2012 to 152.14.136.96. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions 124313-2 Velasco et al. J. Chem. Phys. 124, 124313 ͑2006͒ TABLE I. Exponents and coefficients for the ANO Rydberg functions of CH4. Exponents Coefficients s functions 0.024 624 0.643 460 61 −0.484 662 19 0.398 719 24 −2.647 136 64 2.525 649 54 −3.465 605 36 0.011 253 0.404 349 98 −0.712 090 19 0.772 597 87 4.176 787 24 −7.584 206 82 14.914 072 09 0.005 858 −0.010 542 90 0.865 643 09 −1.542 392 22 0.228 487 77 8.391 958 96 −33.046 699 03 0.003 346 −0.013 849 78 0.709 028 79 −1.664 636 45 −3.312 898 40 0.330 971 11 42.958 714 38 0.002 048 −0.018 619 79 0.125 740 29 1.823 273 38 −0.541 457 75 −6.720 791 84 −30.256 212 75 0.001 324 0.018 594 31 −0.106 369 66 0.691 098 47 2.409 889 42 0.716 886 50 9.335 159 66 0.000 893 −0.008 356 44 0.052 799 37 0.072 064 96 −0.577 743 20 2.966 537 48 −2.517 196 34 0.000 624 0.002 060 17 −0.013 895 29 −0.006 172 52 0.202 403 10 −0.187 192 14 2.174 173 74 p functions 0.042 335 0.260 989 76 −0.298 984 13 0.406 373 81 −0.953 532 56 1.168 589 95 −2.456 452 22 0.019 254 0.573 272 90 −0.526 027 65 0.363 939 25 0.240 233 70 −1.350 118 20 7.383 015 97 0.009 988 0.173 828 54 0.183 843 95 −0.647 425 47 1.784 307 66 −1.642 588 13 −11.721 091 95 0.005 689 0.124 604 77 0.577 778 35 −0.829 451 80 −0.639 056 71 4.450 297 11 11.318 832 81 0.003 476 −0.107 929 32 0.446 111 57 0.103 692 69 −1.845 988 61 −1.342 613 08 −8.376 823 52 0.002 242 0.088 962 36 −0.017 309 35 0.857 494 45 0.340 835 52 −2.402 010 36 9.579 264 08 0.001 511 −0.050 559 29 0.044 524 17 0.331 726 39 0.545 437 57 −0.592 669 40 −11.000 030 88 0.001 055 0.013 686 73 −0.013 842 82 0.080 912 09 0.607 431 36 2.226 695 51 5.232 893 81 d functions 0.060 540 0.076 346 20 −0.075 628 58 0.116 857 13 −0.566 346 60 1.095 559 18 −1.587 747 39 0.027 446 0.268 016 70 −0.250 643 94 0.288 435 55 −0.600 150 00 −0.849 091 89 3.571 929 64 0.014 204 0.399 193 53 −0.328 994 50 0.135 178 16 0.673 102 42 −1.045 042 25 −3.713 946 49 0.008 077 0.299 443 15 −0.053 669 72 −0.022 612 53 1.026 542 02 1.473 938 79 1.044 364 50 0.004 927 0.091 062 97 0.427 342 95 −1.057 026 76 −0.801 794 31 0.401 226 59 0.397 577 60 0.003 175 0.014 485 92 0.472 793 16 0.449 547 94 −0.241 505 91 0.618 884 30 3.566 477 61 0.002 137 −0.001 558 80 0.184 140 58 −0.712 288 86 −1.168 065 16 −4.083 173 52 −7.144 409 67 0.001 491 0.000 446 97 0.030 547 13 1.510 554 22 1.439 373 21 2.886 124 95 3.792 400 33 + CH4 from the tetrahedral configuration to a D2d geometry is couplings associated to the excitation to low-lying Rydberg energetically favored.
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