Russell D. Johnson

Two photon resonance enhanced multiphoton ionization spectroscopy of gas phase O2 a 1Δg between 305–350 nm

The resonance enhanced multiphoton ionization (REMPI) spectrum of O2 a 1Δg between 305 and 350 nm is reported. The spectrum is compared to the REMPI spectrum of ground state molecular oxygen in the same electronic energy region, and a number of differences in the respective spectra are observed. Detection limits for O2 a 1Δg by REMPI is calculated to be 5×109 molecule cm−3 in the ion source of the mass spectrometer.

Scaling Factors and Uncertainties for ab Initio Anharmonic Vibrational Frequencies.

To predict the vibrational spectra of molecules, ab initio calculations are often used to compute harmonic frequencies, which are usually scaled by empirical factors as an approximate correction for errors in the force constants and for anharmonic effects. Anharmonic computations of fundamental frequencies are becoming increasingly popular. We report scaling factors, along with their associated uncertainties, for anharmonic (second-order perturbation theory) predictions from HF, MP2, and B3LYP calculations using the 6-31G(d) and 6-31+G(d,p) basis sets. Different scaling factors are appropriate for low- and high-frequency vibrations. The method of analysis is based upon the Guide to the Expression of Uncertainty in Measurement, published by the International Organization for Standardization (ISO). The data used are from the Computational Chemistry Comparison and Benchmark Database (CCCBDB), maintained by the National Institute of Standards and Technology, which includes more than 3939 independent vibrations for 358 molecules.

Multiphoton ionization of SiH3 and SiD3 radicals: Electronic spectra, vibrational analyses of the ground and Rydberg states, and ionization potentials

The electronic spectra of silyl radicals, SiH3 and SiD3, were observed between 310 and 430 nm (46 000–64 000 cm−1) by resonance enhanced multiphoton ionization (REMPI) mass spectroscopy. The spectra were generated through a 2+1 REMPI mechanism. Two Rydberg series originating from planar, D3h point group states were observed. One series, of quantum defect δ=1.45(2), is comprised of the E 2A‘2(4p), J 2A‘2(5p), and M 2A‘2(6p) Rydberg states which have origins at ν0–0 =48 438, 56 929, and 60 341 cm−1 in SiH3 and at ν0–0 =48 391, 56 874, and 60 267 cm−1 in SiD3. In SiD3 theP 2A‘2(7p) Rydberg origin was observed at ν0–0 =62 002 cm−1. The H, K, and N states observed in the SiD3 spectrum comprise the second Rydberg series, δ=2.09, and were tentatively assigned as ns 2A’1 Rydberg states (n=5, 6, 7). The K and N origins were observed at ν0–0 =58 417 and 61 005 cm−1. A fit of the Rydberg formula to the np 2A‘2(n≥5) origins found the adiabatic ionization potential of the SiH3 and SiD3 radicals to be IPa=8.13...

The electronic spectrum of the GeH3 radical

The germyl (GeH3) radical has been observed by resonance enhanced multiphoton ionization (REMPI) spectroscopy in the region of 370–430 nm. The spectrum arises from two‐photon resonances with the 5p 2A‘2 (D3h) Rydberg state that possesses an origin at 419.1 nm (ν0–0 =47 705 cm−1). A vibrational progression of 756 cm−1 was assigned to the ‘‘umbrella’’ mode, ν2. The observed X 2A1 (C3v) v‘2=2 to v‘2=0 vibrational interval is 663 cm−1 which leads to an estimated barrier to inversion of 1530 cm−1.

Electronic spectra of the heteroisotopic CH2D and CHD2 radicals by resonance enhanced multiphoton ionization

The 3p 2B1←←X 2B1 bands of CH2D and CHD2 radicals were observed between 305 and 335 nm by mass resolved, 2+1 resonance enhanced multiphoton ionization spectroscopy. Spectroscopic constants were found for the 3p 2B1 Rydberg state of the CH2D radical (ν00=59 940 cm−1, ν1a1 CH2 stretch=2995 cm−1, ν2a1 CD stretch=2220 cm−1, ν4b1 out‐of‐plane large amplitude (OPLA)=1260 cm−1, ν5b2 CH2 asymmetric stretch=3055 cm−1, ν6b2 CD bend=1115 cm−1) and of the CHD2 radical (ν00=59 920 cm−1, ν1a1 CH stretch=3040 cm−1, ν2a1 CD2 stretch=2150 cm−1, ν4b1 OPLA=1165 cm−1, ν6b2 CH bend=1210 cm−1). Vibrational frequencies calculated by ab initio theory agree well with the experimental data.

A new eletronic spectrum of the SiH3 radical observed using multiphoton ionization spectroscopy

Abstract The silyl radical, SiH 3 , was observed between 365 and 410 nm using resonance-enhanced multiphoton ionization spectroscopy. The spectrum arises from two-photon resonance states which lie between 49229 and 54014 cm −1 . A vibrational progression interval of ≈ 800 cm −1 was assigned to the resonant intermediate state symmetric deformation mode, ν 2 .

Anharmonic Vibrational Frequency Calculations Are Not Worthwhile for Small Basis Sets.

Anharmonic calculations using vibrational perturbation theory are known to provide near-spectroscopic accuracy when combined with high-level ab initio potential energy functions. However, performance with economical, popular electronic structure methods is less well characterized. We compare the accuracy of harmonic and anharmonic predictions from Hartree-Fock, second-order perturbation, and density functional theories combined with 6-31G(d) and 6-31+G(d,p) basis sets. As expected, anharmonic frequencies are closer than harmonic frequencies to experimental fundamentals. However, common practice is to correct harmonic predictions using multiplicative scaling. The surprising conclusion is that scaled anharmonic calculations are no more accurate than scaled harmonic calculations for the basis sets we used. The data used are from the Computational Chemistry Comparison and Benchmark Database (CCCBDB), maintained by the National Institute of Standards and Technology, which includes more than 3939 independent vibrations for 358 molecules.

Uncertainty associated with virtual measurements from computational quantum chemistry models

A value for the measurand determined from a computational model is frequently referred to as a virtual measurement to distinguish it from a physical measurement, which is determined from a laboratory experiment. Any measurement, physical or virtual, is incomplete without a quantitative statement of its associated uncertainty. The science and technology of making physical measurements and quantifying their uncertainties has evolved over many decades. In contrast, the science and technology of making virtual measurements is evolving. We propose an approach for quantifying the uncertainty associated with a virtual measurement of a molecular property determined from a computational quantum chemistry model. The proposed approach is based on the Guide to the Expression of Uncertainty in Measurement, published by the International Organization for Standardization, and it uses the Computational Chemistry Comparison and Benchmark Database maintained by the National Institute of Standards and Technology.

Multiphoton ionization of SiH3 and SiD3 radicals. II. Three‐photon resonance‐enhanced spectra observed between 450 and 610 nm

The electronic spectra of silyl radicals, SiH3 and SiD3, were observed between 450 and 610 nm (49 200‐65 200 cm−1 ) by resonance enhanced multiphoton ionization (REMPI) spectroscopy. The spectra were produced through a 3+1 REMPI mechanism. Spectra of four new planar Rydberg states were observed and assigned. In SiH3 the observed states and spectroscopic constants are D 2A’1(3d)■ T0=49 787(30), ω2=810(31) cm−1; I’ (4d): T0=56 253(30), ω2=814(25) cm−1; J’ (4d): T0=57 726(30), ω2=835(26) cm−1; and L (5d): T0=59 615(30) cm−1, ω2=839(26) cm−1. In SiD3 the observed states and spectroscopic constants are D 2A1(3d): T0=49 685(30), ω2=600(28) cm−1; I’ (4d):T0=56 205(30), ω2=600(17) cm−1; and J’ (4d):T0=57 840(30), ω2=603(20) cm−1. The differences between 2+1 and 3+1 REMPI spectra are explained by a Δl rule, where l is the electronic orbital angular momentum. The H, K, and N states observed in the 2+1 REMPI spectrum of SiD3 spectrum and reported in J. Chem. Phys. 91, 3340 (1989) are reassigned as the H 2...

New electronic states of NH and ND observed from 258 to 288 nm by resonance enhanced multiphoton ionization spectroscopy

Three new electronic states of NH and ND (imidogen radical) have been observed by REMPI (resonance enhanced multiphoton ionization) spectroscopy in the region of 258 through 288 nm and assigned. The NH (ND) was produced by the photolysis of HN3 (DN3) (hydrazoic acid) in the same wavelength region. The observed two‐photon transitions are from the a 1Δ state to 3p Rydberg states. Transitions were also observed from the a 1Δ state to the d 1Σ+ state. The new state assignments are: f 1Π (3pσ) at 86 378 cm−1, g 1Δ (3pπ) at 88 140 cm−1, and h 1Σ (3pπ) at 89 531 cm−1. Rotational constants (B and D) and, where possible, vibrational spacings for the thirteen observed bands are also determined.