berkowitz_1987

J. Berkowitz, J. P. Greene, H. Cho and B. Ruscic, “The ionization potentials of CH4 and CD4”, J. Chem. Phys. 86, 674 (1987)

Source record · Catalogue · Review register

DOI: 10.1063/1.452268

Source kind: measurement

Recorded technique: Photoionisation mass spectrometry: photoion yield curves of CH4+ and CD4+ near threshold, the target gases cooled to 78 K, wavelength resolution 0.28 A FWHM, target pressure about 1e-5 Torr (the same for both gases); the threshold read from the yield curve (Fig. 1) (pp. 674-675).

Materials in structured entries: CH4

Dataset quantities: ionization_energy

Contents: 1 dataset; 0 formulas.

Curation notes

PDF pages 2-4 = journal pages 674-676 (PDF page 1 is the publisher’s cover sheet). Identity checked on the first page.

CH4 (p. 675): ‘The observed threshold occurs at 983 (+0, -1) A = 12.61 (+0.01, -0.00) eV and is not influenced by temperature’; the yield curve is ‘essentially the same’ as Chupka & Berkowitz’s 1971 (Ref. 5). Extracted as the adiabatic ionisation energy (the paper’s sense: its title and abstract speak of the adiabatic ionisation potentials, and the CD4 value is ‘higher than that of CH4’ by 0.05 +- 0.02 eV). It is the onset of the photoion yield at 78 K: the paper argues (pp. 675-676) that the Boltzmann distribution of rotational levels moves the J’’ = 0 -> J’ = 0 adiabatic value to slightly higher energy than the onset. Its model (C2v CH4+ as a symmetric top, perpendicular transition) puts the linearly extrapolated onset about 43 cm^-1 below the rotationally adiabatic value for CH4+ and 31 cm^-1 for CD4+, ‘the difference between the extrapolated onset and the rotationally adiabatic value is ~0.005 eV for CH4’ (the sign is printed as ‘-0.005’). ‘By choosing the extrapolated onsets, instead of attempting to infer the rotationally adiabatic values, we incur a possible error of ~0.001 eV for the difference’, and ‘rotational effects are too small to influence the present results’. So 12.61 eV is the onset; the rotationless value implied by the paper’s model is about 12.615 eV (not printed). The printed uncertainty is asymmetric, +0.01 / -0.00 eV; it is recorded as the one sigma column value 0.01, level not stated. Also: the step heights (second to first) are <= 3 for CH4.

CD4 (not a curated species; notes): adiabatic IP ‘979.5 +- 1 A = 12.65_8 +- 0.015 eV’ (abstract, p. 675), attributed to zero-point energy differences; the calculated difference IP(CD4) - IP(CH4) is 0.013 eV (Paddon-Row et al. frequencies), 0.056 eV (HF/6-31G* frequencies) and 0.050 eV (scaled by 0.89) (p. 674).

EARLIER VALUES QUOTED (p. 674, notes): PIMS and PES adiabatic values range from 12.55 +- 0.05 to 12.78 eV; Rabalais et al.: ‘below 12.616 eV, and probably ... near 12.51 eV’; CD4/CH4 pairs 12.882 +- 0.008 / 12.704 +- 0.008 (Nicholson 1965), 12.87 +- 0.02 / 12.71 +- 0.02 (Dibeler et al. 1965), 12.83 / 12.75 eV (Brundle, Robin & Basch 1970). Brundle et al.’s first PES band has two broad Jahn-Teller peaks and a shoulder; the splitting is 0.8 eV (CH4+) and 0.7 eV (CD4+) (p. 676). Zero-point energies (p. 674): 27.10 (CH4), 20.10 (CD4) kcal/mol from experiment; calculated 25.1 (CH4+), 18.4 kcal/mol (CD4+).

Recorded bibliographic identity status: VALIDATED_WITH_WARNINGS. This concerns identification of the source; it does not establish the scientific correctness of its contents.

Datasets

berkowitz_1987/adiabatic_CH4

{
  "id": "adiabatic_CH4",
  "material": "CH4",
  "table": "text",
  "quantity": "ionization_energy",
  "unit": "eV",
  "abscissa": "label",
  "use": "primary",
  "parameters": {
    "kind": "adiabatic",
    "final_state": "X 2T2 ground state of CH4+ (Jahn-Teller distorted, C2v), onset",
    "temperature_k": 78
  },
  "uncertainty": {
    "column": true,
    "level": "not stated"
  }
}

1 inline points in the source record.

p. 675: ‘The observed threshold occurs at 983 (+0, -1) A = 12.61 (+0.01, -0.00) eV and is not influenced by temperature’ (gas at 78 K). The onset of the photoion yield, taken by the paper as the adiabatic ionisation potential; the rotationally adiabatic (J = 0 -> 0) value lies about 0.005 eV higher by the paper’s model (record notes). The uncertainty is printed asymmetric, +0.01 / -0.00 eV: 0.01 recorded.

Formulas

No formulas are recorded.

Corrections

No dataset errata are recorded in this source record.

Formula corrections, where recorded, appear with their formulas above. Curation notes may also discuss corrections or unresolved interpretation. Consult the review register for the checks and decisions.