Photoionization Efficiency Data Research Articles

Identification of Tetrahydrofuran Reaction Pathways in Premixed Flames

Abstract Premixed low-pressure tetrahydrofuran/oxygen/argon flames are investigated by photoionization molecular-beam mass spectrometry using vacuum-ultraviolet synchrotron radiation. For two equivalence ratios (φ = 1.00 and 1.75), mole fractions are measured as a function of distance from the burner for almost 60 intermediates with molar masses ranging from 2 (H2) to 88 (C4H6O2), providing a broad database for flame modeling studies. The isomeric composition is resolved by comparisons between experimental photoionization efficiency data and theoretical simulations, based on calculated ionization energies and Franck-Condon factors. Special emphasis is put on the resolution of the first reaction steps in the fuel destruction. The photoionization experiments are complemented by electron-ionization molecular-beam mass-spectrometry measurements that provide data with high mass resolution. For three additional flames with intermediate equivalence ratios (φ = 1.20, 1.40 and 1.60), mole fractions of major species and photoionization efficiency spectra of intermediate species are reported, extending the database for the development of chemical kinetic models.

Proton formation in 2+1 resonance enhanced multiphoton excitation of HCl and HBr via (Ω=) Rydberg and ion-pair states

Molecular beam cooled HCl was state selected by two-photon excitation of the V (1) summation operator(0(+)) [v=9,11-13,15], E (1) summation operator(0(+)) [v=0], and g (3) summation operator(-)(0(+)) [v=0] states through either the Q(0) or Q(1) lines of the respective (1,3) summation operator(0(+))<--<--X (1) summation operator(0(+)) transition. Similarly, HBr was excited to the V (1) summation operator(0(+)) [v=m+3, m+5-m+8], E (1) summation operator(0(+)) [v=0], and H (1) summation operator(0(+)) [v=0] states through the Q(0) or Q(1) lines. Following absorption of a third photon, protons were formed by three different mechanisms and detected using velocity map imaging. (1) H(*)(n=2) was formed in coincidence with (2)P(i) halogen atoms and subsequently ionized. For HCl, photodissociation into H(*)(n=2)+Cl((2)P(12)) was dominant over the formation of Cl((2)P(32)) and was attributed to parallel excitation of the repulsive [(2) (2)Pi4llambda] superexcited (Omega=0) states. For HBr, the Br((2)P(32))Br((2)P(12)) ratio decreases with increasing excitation energy. This indicates that both the [(3) (2)Pi(12)5llambda] and the [B (2) summation operator5llambda] superexcited (Omega=0) states contribute to the formation of H(*)(n=2). (2) For selected intermediate states HCl was found to dissociate into the H(+)+Cl(-) ion pair with over 20% relative yield. A mechanism is proposed by which a bound [A (2) summation operatornlsigma] (1) summation operator(0(+)) superexcited state acts as a gateway state to dissociation into the ion pair. (3) For all intermediate states, protons were formed by dissociation of HX(+)[v(+)] following a parallel, DeltaOmega=0, excitation. The quantum yield for the dissociation process was obtained using previously reported photoionization efficiency data and was found to peak at v(+)=6-7 for HCl and v(+)=12 for HBr. This is consistent with excitation of the repulsive A(2) summation operator(12) and (2) (2)Pi states of HCl(+), and the (3) (2)Pi state of HBr(+). Rotational alignment of the Omega=0(+) intermediate states is evident from the angular distribution of the excited H(*)(n=2) photofragments. This effect has been observed previously and was used here to verify the reliability of the measured spatial anisotropy parameters.

Initial Steps of Aromatic Ring Formation in a Laminar Premixed Fuel-Rich Cyclopentene Flame†

A fuel-rich, nonsooting, premixed laminar cyclopentene flame (phi = 2.0) at 37.6 Torr (50 mbar) is investigated by flame-sampling photoionization molecular-beam mass spectrometry utilizing vacuum-ultraviolet synchrotron radiation. Mole fractions as a function of distance from the burner are measured for 49 intermediates with ion masses ranging from 2 (H2) to 106 (C8H10), providing a broad database for flame modeling studies. The isomeric composition is resolved for most species, and the identification of several C4Hx, C7H6, and C7H8 isomers is discussed in detail. The presence of C5H5CCH/C5H4CCH2 and cycloheptatriene is revealed by comparisons between flame-sampled photoionization efficiency data and theoretical simulations, based on calculated ionization energies and Franck-Condon factors. This insight suggests a new potential molecular- weight growth mechanism that is characterized by C5-C7 ring enlargement reactions.

Absolute oscillator strengths for photoabsorption and the molecular and dissociative photoionization of nitric oxide

The absolute dipole oscillator strengths (cross sections) for the photoabsorption of NO, measured from 6–190 eV, are compared with a variety of other published measurements. In the 20–30 eV region the present photoabsorption data, determined by dipole (e,e) spectroscopy, are in excellent agreement with those reported by Gardner et al. using synchrotron radiation. At higher energies good agreement is obtained with data reported by Cole and Dexter out to 190 eV. Photoionization branching ratios and absolute dipole oscillator strengths for molecular single and double photoionization as well as for all dissociative photoionization channels are measured by dipole (e,e + ion) spectroscopy in the equivalent photon energy range up to 80 eV. The photoionization efficiency is also measured from threshold up to 60 eV. The ion time-of-flight mass spectra show clear evidence of processes leading to O + and also to N + with high fragment ion kinetic energies. Earlier determined photoelectron branching ratios are used together with the present photoabsorption and photoionization efficiency data to recalculate partial photoionization oscillator strengths for production of electronic states of NO +. Finally these partial oscillator strengths for production of electronic states of NO + are considered together with those for molecular and dissociative photoionization in an attempt to predict the quantitative dipole ionic breakdown pathways for the interaction of energetic radiation with NO below 60 eV.

Photoionization studies of HgKr and HgXe

Photoionization efficiency data for HgKr+ and HgXe+ have been obtained in the region of 730–1290 Å. The ionization energies (IE) of HgKr and HgXe are determined to be 10.056±0.012 (1233±1.5 Å) and 9.709±0.011 eV (1277±1.5 Å), respectively. Using these values, the known dissociation energies of HgKr and HgXe, and the IE of Hg, the binding energies for the ground-state mercury-rare gas molecular ions are calculated to be 0.393±0.013 eV for HgKr+ and 0.748±0.013 eV for HgXe+. By analyzing the shifts in energy between corresponding autoionization peaks observed in the Kr+, HgKr+, Xe+, and HgXe+ spectra and by assuming the charge induced-dipole interaction to be the dominant interaction at the equilibrium bond distances for HgKr and HgXe, the equilibrium bond distances for HgKr and HgXe are deduced to be 3.98 and 4.23 Å, respectively. The latter values are in excellent agreement with values determined by previous spectroscopic studies.

Molecular beam photoionization study of S2

Higher resolution photoionization efficiency data for the formation of S+2 and S+ from S2 in the wavelength region of 600–1350 Å have been obtained using the supersonic oven beam method. The ionization energy (IE) of S2 is determined to be 9.356±0.002 eV (1325.2±0.3 Å). The measured appearance energy of 14.732±0.005 eV (841.6±0.3 Å) for the dissociative ionization process S2+hν→S++S+e−, together with the IE of S2, yields a value of 5.376±0.005 eV for the bond dissociation energy of S+2. Two Rydberg series converging to the b̃ 4Σ−g state (13.20 eV) of S+2 are observed. Window resonances resolved in the wavelength region of 670–850 Å are assigned as members of the Rydberg series converging to the c̃ 4Σ−u (17.70 eV) and 2Πu (or 2Σ−u) (18.66 eV) states of S+2.

Photoionization study of HgAr

Photoionization efficiency data for HgAr+ have been obtained in the region of 680–1240 Å. The ionization energy of HgAr was determined to be 10.217±0.012 eV. This value allows the calculation of the dissociation energy of HgAr+ to be 0.228±0.017 eV. The relative probabilities for the formation of HgAr+ via the reactions Ar* ⋅ Hg or Hg* ⋅ Ar→ HgAr+ +e− with Ar* and Hg* prepared in high Rydberg states in the energy range of 10.22–15.79 eV were estimated. Although the radii for the 3d and 5s Rydberg ortitals of Ar have similar values, the probabilities for the formation of HgAr+ from Hg ⋅ Ar* with Ar* in the 5s[3/2]01 and 5s′[1/2]01 Rydberg states are substantially greater than those when the Ar* excited atoms are in the 3d[1/2]01, 3s[3/2]01, and 3d′[3/2]01 Rydberg levels. The ratio for the cross sections for the formation of HgAr+ from Hg ⋅ Ar* with Ar* formed in the 3d[1/2]01 and 4d[1/2]01 states, as well as that with Ar* prepared in the 5d[1/2]01 and 6d[1/2]01 states, were found to be consistent with the predictions of the previous impact parameter calculations.

Photoionization study of Hg 2

Photoionization efficiency data for Hg 2 + have been obtained in the region of 650–1400 A. The ionization energy of Hg 2 was determined to be 9.103 ± 0.010 eV. This value allows the calculation of the dissociation energy of Hg + 2 to be 1.40 ± 0.02 eV. By analyzing the differences in energy between corresponding autoionization peaks observed in the Hg + and the Hg 2 + spectra and by assuming the charge induced-dipole interaction to be the dominant interaction between Hg +( 2D 5/2, 3/2) and Hg at the equilibrium bond distance of Hg 2, the equilibrium bond distance for Hg 2 was deduced to 3.35 A.

Molecular beam photoionization study of H 2S

Photoionization efficiency data for H 2S +, S + and HS + have been obtained in the region 645–1190 Å using the molecular beam method. The ionization energy of H 2S was determined to be 10.4607±0.0026 eV (1185.25±0.30 Å). The observed autoionizing vibrational progressions are tentatively assigned to the Rydberg transitions: 5 a 1→ nsa 1 ( n = 5 and 6) and 2 b 2→ nda 1 ( n = 4 and 5).

ChemInform Abstract: HIGHER RESOLUTION PHOTOIONIZATION STUDY OF ACETYLENE NEAR THE THRESHOLD

High resolution (0.14 A FWHM) photoionization efficiency data for C2H2+ near the threshold (1059–1090 A) have been obtained using the molecular beam method. The positions of the autoionization features resolved on the v2=0 vibrational step of C2H2+(X 2Πu) were found to be consistent with those calculated from the two Rydberg equations νn=93 770−[R/(n−0.95)2] cm−1, νn=93 770−[R/(n−0.36)2] cm−1, where R and n are the Rydberg constant and the principal quantum number, respectively.

Higher resolution photoionization study of acetylene near the threshold

High resolution (0.14 Å FWHM) photoionization efficiency data for C2H2+ near the threshold (1059–1090 Å) have been obtained using the molecular beam method. The positions of the autoionization features resolved on the v2=0 vibrational step of C2H2+(X̃ 2Πu) were found to be consistent with those calculated from the two Rydberg equations νn=93 770−[R/(n−0.95)2] cm−1, νn=93 770−[R/(n−0.36)2] cm−1, where R and n are the Rydberg constant and the principal quantum number, respectively.

Photoionization study of CO2, N2O dimers and clusters

Photoionization efficiency data for (CO2)+2–4 and (N2O)+2 in the region 600–1020 Å have been obtained using the molecular beam method. The ionization energies (I.E.) of (CO2)2, (CO2)3, (CO2)4, and (N2O)2 were measured to be 13.32±0.02 eV (930.5±1.5 Å), 13.24±0.02 eV (936.5±1.5 Å), 13.18±0.02 eV (941±1.5 Å), and 12.35±0.02 eV (1004±1.5 Å), respectively. Using these values, the known I.E.’s for CO2 and N2O, and the estimated binding energies for (CO2)2 and (N2O)2, the dissociation energies for CO+2⋅CO2, (CO2)+2⋅CO2, (CO2)+3⋅CO2, and N2O+⋅N2O were deduced to be 11.8±1.0, 3.3±1.4, 2.8±1.4, and 13.1±0.9 kcal/mol, respectively. The ion–molecule half reaction N2O+⋅N2O→N3O+2+N was observed. The analysis of the photoionization efficiency curves for (CO2)+2 and (N2O)+2 suggests electronic predissociation or direct dissociation might be important dissociation mechanisms for the CO*2(n)⋅CO2 and N2O*(n)⋅N2O excited Rydberg dimers.

The photoionization of allyl chloride from onset to 20 eV

Photoionization efficiency data have been obtained for the parent ion and two fragment ions (C 3H 5 + and C 3H 3 +) of allyl chloride. Onset energies are determined for these ions and heats of formation calculated for the fragment ions. The structures of the fragment ions are discussed in relation to their heats of formation.

Molecular beam photoionization study of OCS, (OCS)2, (OCS)3, and OCS⋅CS2

Photoionization efficiency (PIE) data for OCS+ near the threshold (1080–1111 Å) were obtained with a resolution of 0.14 Å (FWHM) using the molecular beam method. The ionization energies (IE) for the 2Π3/2 and 2Π1/2 states of OCS were determined to be 11.1736±0.0015 eV (1109.62±0.15 Å) and 11.2204±0.0015 eV (1104.95±0.15 Å), respectively. These values are found to be in agreement with those obtained in previous photoionization experiments. PIE data for (OCS)2, (OCS)3, and OCS⋅CS2 have also been obtained in the wavelength region 750–1270 Å. The observed IE’s for (OCS)2, (OCS)3, and OCS⋅CS2 are 10.456±0.026, 10.408±0.026, and 9.858±0.024 eV, respectively. From the measured IE’s and the estimated dissociation energies of (OCS)2 and OCS⋅CS2, the binding energies for OCS+⋅OCS and CS+2⋅OCS are deduced to be 17.2±1 and 5.8±1 kcal/mol, respectively. From the shifts in energy of the Rydberg series of CS2 and OCS in the OCS⋅CS2 and (OCS)2 dimers, and from the symmetry of the Rydberg orbitals, possible geometries for OCS⋅CS2 and (OCS)2 are discussed.

ChemInform Abstract: MOLECULAR BEAM PHOTOIONIZATION STUDY OF CARBON DISULFIDE, CARBON DISULFIDE DIMER AND CLUSTERS

A new molecular beam photoionization mass spectrometer which is significantly superior to previous similar instruments in both wavelength resolution and ion intensity is described. Photoionization efficiency (PIE) data for CS2+ near the threshold (1210–1232 and 1075–1130 A) were obtained with a resolution of 0.14 A (FWHM). Rydberg series I, n=16–24, which converges to the spin–orbit excited state, 2Π1/2, of CS2+ was resolved on the first vibrational step in the PIE data. Observation of the autoionization resonance, series I (n=16), indicates that the value for the first ionization energy (I.E.) for CS2 obtained in previous absorption experiments (Ref. 23) is too high. The I.E.’s for the 2Π3/2 and 2Π1/2 states of CS2 were determined to be 10.0685±0.0020 and 10.1230±0.0020 eV, respectively. PIE data for (CS2)n+ with n ranging from 2 to 5 have been obtained in the wavelength region from 650 to 1350 A with this apparatus. The observed I.E.’s for (CS2)2, (CS2)3, (CS2)4, and (CS2)5 are 9.36±0.02, 9.22±0.02, 9.10±0.02, and 9.04±0.02 eV, respectively. From the measured ionization energy, and the estimated dissociation energy (0.05 eV) of (CS2)2, the binding energy for CS2+⋅CS2 is deduced to be 17.5±1 kcal/mol. This is in agreement with the value obtained previously in a high pressure mass spectrometry experiment. The Rydberg series for CS2, which are manifested as autoionization lines in the PIE data for (CS2)2+ in the region 850–960 A, are red shifted in energy by ∼0.075 eV for Rydberg series III and ∼0.14 eV for Rydberg series IV. Adapting the most recent assignment of Rydberg series III and IV by Larzilliere et al. symmetry arguments support a linear or near linear geometry for the carbon disulfide dimer.

Molecular beam photoionization study of carbon disulfide, carbon disulfide dimer and clusters

A new molecular beam photoionization mass spectrometer which is significantly superior to previous similar instruments in both wavelength resolution and ion intensity is described. Photoionization efficiency (PIE) data for CS2+ near the threshold (1210–1232 and 1075–1130 Å) were obtained with a resolution of 0.14 Å (FWHM). Rydberg series I, n=16–24, which converges to the spin–orbit excited state, 2Π1/2, of CS2+ was resolved on the first vibrational step in the PIE data. Observation of the autoionization resonance, series I (n=16), indicates that the value for the first ionization energy (I.E.) for CS2 obtained in previous absorption experiments (Ref. 23) is too high. The I.E.’s for the 2Π3/2 and 2Π1/2 states of CS2 were determined to be 10.0685±0.0020 and 10.1230±0.0020 eV, respectively. PIE data for (CS2)n+ with n ranging from 2 to 5 have been obtained in the wavelength region from 650 to 1350 Å with this apparatus. The observed I.E.’s for (CS2)2, (CS2)3, (CS2)4, and (CS2)5 are 9.36±0.02, 9.22±0.02, 9.10±0.02, and 9.04±0.02 eV, respectively. From the measured ionization energy, and the estimated dissociation energy (0.05 eV) of (CS2)2, the binding energy for CS2+⋅CS2 is deduced to be 17.5±1 kcal/mol. This is in agreement with the value obtained previously in a high pressure mass spectrometry experiment. The Rydberg series for CS2, which are manifested as autoionization lines in the PIE data for (CS2)2+ in the region 850–960 Å, are red shifted in energy by ∼0.075 eV for Rydberg series III and ∼0.14 eV for Rydberg series IV. Adapting the most recent assignment of Rydberg series III and IV by Larzilliere et al. symmetry arguments support a linear or near linear geometry for the carbon disulfide dimer.