Branching Ratios of the N(²D⁰_3/2) and N(²D⁰_5/2) Spin–Orbit States Produced in the State-Selected Photodissociation of N₂ Determined Using Time-Sliced Velocity-Mapped-Imaging Photoionization Mass Spectrometry (TS-VMI-PI-MS)

Abstract

Branching ratios for N( 2 D 0 3/2 ) and N( 2 D 0 5/2 ) produced by predissociation of state selected excited nitrogen molecules in the vacuum ultraviolet region have been measured for the first time. The quantum numbers of the excited nitrogen molecule are defined by selective excitation of the nitrogen molecule in the Franck−Condon region from the ground electronic, 1 Σ g + , vibrational, v″, and rotational, J″ state to an excited E u ′, v′, J′ state with a tunable vacuum ultraviolet, VUV 1 , laser. The neutral atoms produced by predissociation from this excited state are then selectively ionized with a second tunable VUV 2 laser. Measurement of the relative populations of these two atoms formed in their spin−orbit states defines the quantum states for the atomic products. This means that the wave functions of the initial state and knowledge of the relative yields define all the experimental parameters for this series of unimolecular reactions. The ions formed by VUV 2 are mass analyzed with a time-offlight mass spectrometer and detected with a time slice velocity ion imaging mass spectrometer. In this manner, we can determine the recoil velocity associated with the predissociation process. Two different techniques are used to determine the spin−orbit ratios, namely, resonant VUV photoionization (RVUV-PI) spectroscopy and total kinetic energy release (TKER) spectroscopy determined from the image produced when the atoms are selectively ionized by VUV 2 in the interaction region. The TKER spectra obtained from the lines at 110 296.25 and 110 304.96 cm −1 that couple to a newly discovered autoionization line at 129 529.4255 ± 0.0015 cm −1 prove that the lines observed in this region originate from the N( 2 D 0 3/2 ) and N( 2 D 0 5/2 ) atoms. Two other lines in this region at 110 286.20 and 110 299.89 cm −1 originate from the nitrogen N( 4 S 0 3/2 ) that is photoionized in a 1+ 1 VUV−UV resonant multiphoton ionization process. The spin−orbit branching ratios have been evaluated for valence and Rydberg electronic excited states from 104 129.4 to 118 772.1 cm −1 , and it shows that they are independent of the rotational and vibrational quantum numbers. They are not appreciably affected by the symmetry properties of the wave function in the Franck−Condon region of the excited states. In the energy region below 117 153.8 cm −1 the pathways at long internuclear distances appear to determine [N( 2 D 0 3/2 )]/[N( 2 D 0 5/2 )] branching ratios of ∼0.38, ∼0.62, and ∼1.04. At higher energies, TKER and RVUV-PI spectroscopy have been used to show that the average fraction of the N( 2 D 0 3/2 ) and N( 2 D 0 5/2 ) atoms produced in the spin-allowed channels that produce two N( 2 D 0 J ) is 0.85 versus 0.15 for spin-forbidden channels. The importance and need for this information for comparison with theory and applications in astrochemistry are briefly discussed.

Multi-channel photodissociation dynamics of 14N2 in its b′ 1Σ+u(ν = 20) state

“…The VUV-VUV-TS-VMI setup has been described before, 17,25 and detailed quantum state-to-state experimental schemes are similar to those by Jackson and coworkers. 20,26 A pulsed supersonic molecular beam of N 2 was generated through a pulsed valve (Parker, series 9) and then passed through two skimmers before entering the photodissociation/photoionization (PD/PI) region. To generate a rotationally hotter molecular beam so that photodissociation at high rotational levels can be studied, we attached a 5 cm long stainless-steel tube with an inner diameter of 1 mm to the nozzle of the pulsed valve.…”

“…To detect N( 4 S) atoms, probe VUV beam was set at 110299.97 cm −1 to excite them to the intermediate state 2s 2 2p 2 ( 3 P)4d 4 P 5/2 and then N atoms in the intermediate state were ionized by absorbing a second UV or visible photon in the probe VUV beam; N ( 2 D 3/2 ) and N( 2 D 5/2 ) atoms were resonantly excited to a common autoionization state to be ionized with the probe VUV beam set at 110296.25 cm −1 and 110304.96 cm −1 , respectively. 26 The generated 14 N 2 + and N + ions were accelerated to the ion detector via the TS-VMI lens system, and then converted into electrical or optical signals for detection. To measure the (1VUV + 1′UV) photoionization spectra of 14 N 2 , only the pump VUV beam was present in the PD/PI region.…”

The Reaction N(2D) + CH3CCH (Methylacetylene): A Combined Crossed Molecular Beams and Theoretical Investigation and Implications for the Atmosphere of Titan

“…The role of nitrogen atoms or ions (both N + and N 2 + ) as well as electronically excited states of N 2 (as the metastable A 3 Σ + u ) have been considered in all photochemical models. Atomic nitrogen can be formed by an extreme ultraviolet photolysis of N 2 but also by N 2 dissociative photoionization, dissociation via an electron impact, galactic cosmic ray absorption, and N 2 + dissociative recombination. − All these processes can lead to the production of atomic nitrogen in its ground 4 S 3/2 state but also in its first electronically excited 2 D 3/2,5/2 states. The 2 D 3/2,5/2 states are metastable with long radiative lifetimes (13.6 and 36.7 h for 2 D 3/2 and 2 D 5/2 , respectively), and, once formed in the upper atmosphere of Titan above 800 km from the surface, the main destiny of N­( 2 D) is to react with other constituents of Titan’s atmosphere, since its collisional deactivation by N 2 is not efficient.…”

The effect of magnetic field on the tunneling yield of ammonia molecules

“…Photoionization of atoms and molecules is the initial step of many interesting strongfield phenomena such as electron recollision and nonsequential double ionization (NDI) , molecular imaging by the recolliding electrons (MIRE) (PENG et al, 2019), highorder above-threshold ionization (HATI) (BRENNECKE and LEIN, 2018), high-order harmonic generation (HHG) (BRAY et al, 2019). Additionally, the electrons emitted by photoionization contain structural information suitable for orbital imaging (YUAN et al, 2017;CHANG et al, 2019) and nuclear dynamics, as well (ARNOLD et al, 2017). Because of all above mentioned, the ionization mechanism depending on the laser parameters, such as intensity, power, frequency, and wavelength has attracted considerable interests of both experimental and theoretical groups.…”