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 <https://github.com/LupoLab/optmat/blob/main/src/optmat/data/sources/berkowitz_1987.toml>`__ · :doc:`Catalogue </the-data/sources>` · :doc:`Review register </register>`

DOI: `10\.1063/1\.452268 <https://doi.org/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
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. code-block:: text

   {
     "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 :doc:`review register </register>` for the checks and decisions.