Search

The epoch of first star formation in the early Universe was dominated by simple atomic and molecular species consisting mainly of two elements: hydrogen and helium. Gaining insight into this constitutive era requires a thorough understanding of molecular reactivity under primordial conditions. We used a cryogenic ion storage ring combined with a merged electron beam to measure state-specific rate coefficients of dissociative recombination, a process by which electrons destroy molecular ions. We found a pronounced decrease of the electron recombination rates for the lowest rotational states of the helium hydride ion (HeH+), compared with previous measurements at room temperature. The reduced destruction of cold HeH+ translates into an enhanced abundance of this primordial molecule at redshifts of first star and galaxy formation. [ABSTRACT FROM AUTHOR]

Electronic stopping power of helium ions in a semiconductor material ZnSe has been investigated through non-adiabatic dynamics simulations at energies of a few keV under channeling condition. The stopping power is predicted to be proportional to velocity for the trajectory along middle axis of a 〈 1 1 0 〉 channel, as expected for the linear response theory accounts for election-hole pair creation. While for the off-center channeling trajectory, a counterintuitive of electronic stopping power versus velocity is observed. Our study, presented herein, finds a non-trivial connection between charge transfer and the force experienced by the projectile. Charge transfer can produce, throughout the collision process, additional force by continuously forming and breaking instantaneous chemical bonds between the projectile and the neighboring host atoms. [ABSTRACT FROM AUTHOR]

Analytic consideration of the Bohr–Oppenheimer (BO) potential curves for diatomic molecules is proposed: accurate analytic interpolation for a potential curve consistent with its rovibrational spectra is found. It is shown that in the BO approximation for four lowest electronic states 1 s σ g and 2 p σ u , 2 p π u and 3 d π g of H 2 + , the ground state X 2 Σ + of HeH and the two lowest states 1 Σ g + and 3 Σ u + of H 2 , the potential curves can be analytically interpolated in full range of internuclear distances R with not less than 4–5–6 s.d. Approximation based on matching the Laurant-type expansion at small R and a combination of the multipole expansion with one-instanton type contribution at large distances R is given by two-point Padé approximant. The position of minimum, when exists, is predicted within 1% or better. For the molecular ion H 2 + in the Lagrange mesh method, the spectra of vibrational, rotational and rovibrational states ( ν , L ) associated with 1 s σ g and 2 p σ u , 2 p π u and 3 d π g potential curves are calculated. In general, it coincides with spectra found via numerical solution of the Schrödinger equation (when available) within six s.d. It is shown that 1 s σ g curve contains 19 vibrational states ( ν , 0 ) , while 2 p σ u curve contains a single one ( 0 , 0 ) and 2 p π u state contains 12 vibrational states ( ν , 0 ) . In general, 1 s σ g electronic curve contains 420 rovibrational states, which increases up to 423 when we are beyond BO approximation. For the state 2 p σ u the total number of rovibrational states (all with ν = 0 ) is equal to 3, within or beyond Bohr–Oppenheimer approximation. As for the state 2 p π u within the Bohr–Oppenheimer approximation the total number of the rovibrational bound states is equal to 284. The state 3 d π g is repulsive, no rovibrational state is found. It is confirmed in Lagrange mesh formalism the statement that the ground state potential curve of the heteronuclear molecule HeH does not support rovibrational states. Accurate analytical expression for the potential curves of the hydrogen molecule H 2 for the states 1 Σ g + and 3 Σ u + is presented. The ground state 1 Σ g + contains 15 vibrational states ( ν , 0 ) , ν = 0 – 14 . In general, this state supports 301 rovibrational states. The potential curve of the state 3 Σ u + has a shallow minimum: it does not support any rovibrational state, it is repulsive. [ABSTRACT FROM AUTHOR]

High-order harmonic generation from the asymmetric helium hydrogen molecular ion exposed to a few-cycle chirped laser field is theoretically investigated by numerically solving the reduced one-dimension non-Born–Oppenheimer time dependent Schrödinger equation. The numerical results show that the plateau of the high-order harmonic spectrum is dramatically broadened and smoother with the introduction of the chirp. Further study presents that only the short quantum path contributes to harmonic emission and the long quantum path disappears in the chirped laser. In addition, an 80-as isolated attosecond pulse is obtained by superposing the harmonics from the 100th to the 130th order in the chirped field. The time-frequency distributions and the electronic probability distributions are presented to explain the underlying physical mechanism. [ABSTRACT FROM AUTHOR]

More than 80 excited electronic states of the hydrohelium ion HeH+of1, 3Σ+,1, 3Π,1, 3Δ,1, 3Φ and1, 3Γ symmetry have been calculatedab initioup ton= 6 for internuclear distances ranging from 0.5 to 100 bohr. The computations involve a configuration interaction (CI) treatment based on a home-made suite of programs that uses special basis sets designed for the representation of molecular Rydberg states. The results are compared with previous computations where these are available (up ton= 4), and it is found that except for the very lowest excited states, the present energies are consistently lower than those obtained previously, with an average lowering corresponding to several hundred cm−1. It is shown that with the exception of its ground state, HeH+is an effective one-electron system having an overall electronic structure similar to H+2. The interaction of the excited electron with the He+1score electron causes small singlet–triplet splittings to appear and ℓ-mixing interactions to occur, that are not present in H+2. [ABSTRACT FROM AUTHOR]

Complexes of the triatomic hydrogen ion with helium were synthesised in a low-temperature 22-polerfion trap at He number densities of up to 1016cm−3. Absolute ternary rate coefficients for sequentially attaching He atoms have been determined from the growth of complexes with increasing storage time. The number of helium-tagged ions is significantly reduced when increasing the nominal temperature from 4 to 25 K. Competition between attachment and dissociation via collisions leads to stationary Hen–H+3(nup to 9) distributions. State-specific excitation of the trapped H+3ions via IR transitions significantly reduces the formation of complexes. Tuning the laser to Δv2= 1 transitions in the range of 2726 cm−1leads to LIICG lines, i.e., to spectra caused bylaser-induced inhibition of complex growth. In addition, almost 100 lines have been found between 2700 and 2765 cm−1, which are attributed tolaser-induced dissociationof thein situformed He–H+3complex ions. These lines are not yet assigned; however, their absorption strength, statistics and predissociation lifetimes provide interesting information on both the stable complexes as well as on scattering resonances in low-energy H+3+He collisions. New calculations of the potential energy surface will help to analyse the dissociation spectrum. There are some indications that para-H+3is enriched under the conditions of the present experiment. [ABSTRACT FROM AUTHOR]

Wepresent the adiabatic quantum dynamics of the proton-transferreaction H(2S) + HeH+(X1Σ+) → H2+(X2Σg+) + He(1S) on the HeH2+X̃2Σ+RMRCI6 (M = 6) PES of C. N. Ramachandran et al. (Chem. Phys. Lett.2009, 469, 26). We considerthe HeH+molecule in the ground vibrational–rotationalstate and obtain initial-state-resolved reaction probabilities andthe ground-state cross section σ0and rate constant k0by propagating time-dependent, coupled-channel,real wavepackets (RWPs) and performing a flux analysis. Three differentwavepackets are propagated to describe the wide range of energiesexplored, from cold (0.0001 meV) to hyperthermal (1000 meV) collisionenergies, and in a temperature range from 0.01 to 2000 K. We compareour time-dependent results with the time-independent ones by D. DeFazio and S. Bovino et al., where De Fazio carried out benchmark coupled-channelcalculations whereas Bovino et al. employed the negative imaginarypotential and the centrifugal-sudden approximations. The RWP crosssection is in good agreement with that by De Fazio, except at thelowest collision energies below ∼0.01 meV, where the former is larger than the latter.However, neither the RWP and De Fazio results possess the huge resonancein probability and cross section at 0.01 meV, found by Bovino et al.,who also obtained a too low σ0at high energies.Therefore, the RWP and De Fazio rate constants compare quite well,whereas that by Bovino et al. is in general lower. [ABSTRACT FROM AUTHOR]

The article discusses views of astronomers on helium hydride ion. Topics discussed include protons and electrons which acted as catalysts for gas-phase chemical reactions, impact of theoretical calculations on future of astrochemistry, and evolution of chemical species which were under interstellar medium conditions.