Initial Vibrational Level Research Articles

Exhaustive State-to-State Cross Sections and Rate Coefficients for Inelastic N2-N2 Collisions using QCT Combined with Neural Network Models.

Using the quasi-classical trajectory method, we systematically studied the state-to-state vibrational relaxation process of N2(v1) + N2(v2) collisions over a wide temperature range (5000-30,000 K). Different temperature dependencies of the single- and multiquantum VV and VT events in various (v1,v2) collisions are captured, with the dominant channel being related to the initial vibrational energy levels (vmax = 50). At a specified relative translational energy, there is a monotonic relationship of the VT cross sections with the vibrational energy level, particularly in high-energy collisions. Additionally, we constructed well-trained neural network models (R-values reaching 0.99) using limited quasi-classical trajectory (QCT) data sets, which can be used to predict the state-to-state cross sections and rate coefficients of the VV processes N2(v1) + N2(v2) → N2(v1 - Δv) + N2(v2 + Δv) and VT processes N2(v1) + N2(v2) → N2(v1 - Δv) + N2(v2) (Δv = ±1, ±2, ±3) for collisions with arbitrary initial vibrational states. This work not only significantly reduces computational resources but also serves as a reference for the study of the state-to-state dynamics of all four-atom collision systems in hypersonic flows.

Quantum dynamical study on 15N ＋ 14N14N isotope exchange reactions at high energies

In this study, we conduct quantum dynamical calculations at relatively high energies on the exchange reaction 15N ＋ 14N14N →15N14N ＋ 14N on its ground electronic N3 (4A′′) potential energy surface (PES) utilizing the time-dependent wave packet (TDWP) propagation method. Our primary focus is on analyzing the reaction mechanism for 15N ＋ ortho-14N14N (j = 0) and para-14N14N (j = 1) cases. We also explore the influence of varying J (up to 120) of the three-body system and the initial vibrational level (ν = 1, 5, 10) of the diatom on the reaction probability (RP) for ortho and para configurations.

Semiclassical Multistate Dynamics for Six Coupled 5A' States of O + O2.

Dynamics simulations of high-energy O2-O collisions play an important role in simulating thermal energy content and heat flux in flows around hypersonic vehicles. To carry out such dynamics simulations efficiently requires accurate global potential energy surfaces and (in most algorithms) state couplings for many energetically accessible electronic states. The ability to treat collisions involving many coupled electronic states has been a challenge for decades. Very recently, a new diabatization method, the parametrically managed diabatization by deep neural network (PM-DDNN), has been developed. The PM-DDNN method uses a deep neural network architecture with an activation function parametrically dependent on input data to discover and fit the diabatic potential energy matrix (DPEM) as a function of geometry, and the adiabatic potential energy surfaces are obtained by diagonalization of a small matrix with analytic matrix elements. Here, we applied the PM-DDNN method to the six lowest-energy potential energy surfaces in the 5A' manifold of O3 to perform simultaneous diabatization and fitting; the data are obtained by extended multistate complete-active-space second-order perturbation theory. We then used the adiabatic surfaces for dynamics calculations with three methods: coherent switching with decay of mixing (CSDM), curvature-driven CSDM (κCSDM), and electronically curvature-driven CSDM (eκCSDM). The κCSDM calculations require only adiabatic potential energies and gradients. The three dynamical methods are in good agreement. We then calculated electronically nonadiabatic, electronically inelastic, and dissociative cross sections for seven initial collision energies, five initial vibrational levels, and four initial rotational levels. Trends in the electronically inelastic cross sections as functions of the initial collision energy and vibrational level were rationalized in terms of the coordinate ranges where the gaps between the second and third potential energy surfaces are small.

Multistate electronic quenching: Nonadiabatic pathways in NOA2Σ+ + O2X 3Σg - scattering.

The quenching of NOA2Σ+ with O2 as a collisional partner is important for combustion and atmospheric processes. There is still a lack of theoretical understanding of this event, especially concerning the nature of the different quenching pathways. In this work, we provide potential energy surfaces (PESs) of 20 electronic states of this system. We computed the spin-doublet and spin-quartet PESs using SA-CASSCF and XMS-CASPT2. We find two potential quenching pathways. The first one (Q1) is a two-step orientation-specific process. The system first undergoes an electron transfer (NO+X1Σ+ + O2 -X 2Πg) at short distances, before crossing to lower neutral states, such as NOX2Π + O2a 1Δg, O2b 1Σg +, O2X 3Σg -, or even 2 O(3P). The second quenching pathway (Q2) is less orientation-dependent and should be sudden without requiring the proximity conditioning Q1. The Q2 cross section will be enhanced with increasing initial vibrational level in both O2 and NO. It is responsible for the production of NOX2Π with higher O2 excited states, such as O2c 1Σu -, A'3Δu, or A 3Σu +. Overall, this work provides a first detailed theoretical investigation of the quenching of NOA2Σ+ by O2X 3Σg - as well as introduces a weighting scheme generally applicable to multireference, open-shell bimolecular systems. The effect of spin-multiplicity on the different quenching pathways is also discussed.

Infrared frequency comb spectroscopy of CH2I2: Influence of hot bands and pressure broadening on the ν1 and ν6 fundamental transitions.

Direct frequency comb spectroscopy was utilized to measure the vibrational absorption spectrum of diiodomethane, CH2I2, from 2960 to 3125cm-1. The data were obtained using a CH2I2 concentration of (6.8 ± 1.3) × 1015moleculecm-3 and a total pressure of 10-300mbar with either nitrogen or argon as the bath gas. The rovibrational spectra of two fundamental transitions, ν6 and ν1, were recorded and analyzed. We suggest that a significant contribution to the observed congested spectra is due to the population in excited vibrational states of the low energy ν4 I-C-I bend, resulting in transitions 61 04n n and 11 04n n, where the integer n is the initial vibrational level v = 1-5. PGOPHER was used to fit the experimental spectrum, allowing for rotational constants and other spectral information to be reported. In addition, it was found that the peak widths for the observed transitions were limited by pressure broadening, resulting in a pressure broadening parameter of (0.143 ± 0.006) cm-1 atm-1 by N2 and (0.116 ± 0.006) cm-1 atm-1 by Ar. Further implications for other dihaloalkane infrared spectra are discussed.

Quantum mechanical investigation of N + N2 collision: state-to-state non-reaction and exchange reaction probabilities and rate constants

We present a state-to-state dynamical calculation on the exchange reaction N + N2 → N2 + N and the non-reaction N + N2 → N + N2 based on the potential energy surface published by Mankodi et al. The calculation is performed using the time-independent quantum reaction scattering program. The reactivity of both reaction processes is discussed by reaction properties of vibrational quantum numbers v = 0–3 and rotational quantum numbers j = 0–32 (such as cumulative reaction probability, state-to-state reaction probabilities, and cross sections of N exchange, state-to-state rate constants for both reactions). The threshold energy of the exchange reaction can decrease with the decrease of vibrational excitation or the increase of rotational excitation. By using the J-shifting approximation, rate constants are reported for both reactions. The comparison of the presented total rate constant of the N + N2 exchange reaction with the previous results shows that the quantum effect is not negligible at low temperatures. For the exchange reaction, the rate constant at 500 K decreases by about 10 orders of magnitude when the vibrational level of N2 increases from 0 to 7, indicating that the rate constants are sensitive to the initial vibrational level of N2 at low temperatures. For non-reactive collision, the rate constants have little effect on the initial ro-vibrational levels of N2 at low temperatures.

N + O2(v) collisions: reactive, inelastic and dissociation rates for state-to-state vibrational kinetic models

The reactive, inelastic and dissociation vibrationally-detailed rate coefficients of the N + O2 collision processes, previously calculated by means of a QCT method, have been interpolated as a function of the initial and final vibrational levels and of the temperature, in the range 1000–20000 K. These rates have been implemented in a N2/N/O2/O/NO mixture state-to-state kinetic model. The model has been tested in a couple of applications. The first consists in its time evolution, for 10−3 s, from an equilibrium composition at 4000/3000 K towards a new composition obtained by suddenly decreasing the temperature to 1000 K. This is a preliminary study that could open the way to a new approach in the investigation of some biomedical applications governed by plasma sources at atmospheric pressure. The second test regards the boundary layer formed around the nose of a hypersonic vehicle in the framework of the fast transportation and space tourism applications.

Ultrafast quantum imaging in the dissociation of H2+ via the induced conical intersection of two lowest adiabatic states by strong field laser pulses

• A non-Born-Oppenheimer time-dependent Schrödinger equation is numerically solved to study the dissociative-ionization of H 2 + molecule subjected to strong field six-cycle laser pulses ( I = 4 × 10 14 W / cm 2 , λ = 800 nm ) . • Analysis of the time evolution of the H 2 + nuclear wave packet is performed using Floquet formalism in the adiabatic representation to gain newly insightful images of the nonlinear processes such as vibrational trapping, above-threshold dissociation, tunneling, and bond-softening of the laser-driven nuclear wave packet on two lowest field-dressed adiabatic potential energy curves. • Starting from an initial state-selected vibrational level of H 2 , the images enable ones to fully comprehend the dissociative-ionization dynamics of H 2 + , notably in the vicinity of a conical intersection up to eventually entering the two exit channels with the main one producing fragments H + H + via the H 2 + ( 2 ∑ u + ) exit channel. • The direct treatment of the electronic-nuclear wave packet of H 2 enables one to image the one-electron nuclear localization of H 2 + on the right or left of field-distorted double-well Coulomb potential to reveal the influence of the field on the charge-resonance enhanced ionization coupled with the other nonlinear processes. A non-Born–Oppenheimer time-dependent Schrödinger equation is numerically solved to study the dissociative-ionization of H 2 + subjected to strong field six-cycle laser pulses ( I = 4 × 10 14 W / cm 2 , λ = 800 nm ) . Analysis of the evolution of H 2 + nuclear wave packet is performed using Floquet formalism in the adiabatic representation to gain newly insightful images of the nonlinear processes in the vicinity of a conical intersection of two lowest field-dressed adiabatic states. The coherent superposition of the states leads to the right or left localization of H 2 + on the field-distorted double-well Coulomb potential.

Isotopic and vibrational-level dependence of H2 dissociation by electron impact

The low-energy electron-impact dissociation of molecular hydrogen has been a source of disagreement between various calculations and measurements for decades. Excitation of the ground state of H$_2$ into the dissociative $b ^3\Sigma_u^+$ state is now well understood, with the most recent measurements being in excellent agreement with the molecular convergent close-coupling (MCCC) calculations of both integral and differential cross sections (2018 Phys. Rev. A 98 062704). However, in the absence of similar measurements for vibrationally-excited or isotopically-substituted H$_2$, cross sections for dissociation of these species must be determined by theory alone. We have identified large discrepancies between MCCC calculations and the recommended $R$-matrix cross sections for dissociation of vibrationally-excited H$_2$, D$_2$, T$_2$, HD, HT, and DT (2002 Plasma Phys. Contr. F. 44 1263-1276,2217-2230), with disagreement in both the isotope effect and dependence on initial vibrational level. Here we investigate the source of the discrepancies, and discuss the consequences for plasma models which have incorporated the previously recommended data.

Ro-vibrational level dependence of the radiative lifetime of the Na241Σg + shelf state.

We report on calculations-using the LEVEL and BCONT programs by Le Roy, the latter of which is a version modified by B. McGeehan-of the dependence of the radiative lifetime of the Na2 sodium dimer 41Σg + shelf-state on the initial vibrational and rotational level for corresponding quantum numbers of 0 ≤ v ≤ 75 and 0 ≤ J ≤ 90, respectively. We also present experimental lifetime values for 43 < v < 64, averaged over J = 19 and 21, obtained by a delayed pump-probe method using a previously described molecular beam and time-of-flight apparatus. Our calculated results are based on all possible dipole allowed transitions (to the 21Σu +, 1(B)1Πu, and 1(A)1Σu + electronic states) terminating into bound as well as free final states. The shelf of the initial electronic state is a consequence of configuration interaction with the lowest Na+-Na- ion-pair potential and occurs, for the rotationless molecule, at the vibrational level v = 52. From the 41Σg + vibrational ground state to the shelf, the calculated lifetimes increase monotonically by a factor of about 3.8. Beyond around v = 52, depending on rotational excitation, the lifetimes decrease, settling to a value intermediate to the maximum and the minimum at v = 0. Within error bars and in the range available, our experimental data are compatible with these findings. In addition, our calculations reveal unusual and pronounced oscillatory variation of the lifetime with rotational quantum numbers for fixed vibrational levels above-but not below-the shelf. We discuss our findings in terms of the appropriate transition dipole moments and wavefunctions and provide a detailed comparison to recent lifetime calculations of sodium dimer ion-pair states [Sanli et al., J. Chem. Phys. 143, 104304 (2015)].

Geometric phase effects on photodissociation dynamics of diatomics

We investigated the effect of the geometric phase (GP) on photodissociation dynamics at a light-induced conical intersection (LICI) through exact quantum dynamical calculations. By taking the one-photon photodissociation of ionic molecules as an example, we explored the conditions wherein the LICI associated GP affects dissociation dynamics. We found that GP leads to a phase shift between the angular distributions of GP included and GP excluded photofragments. This effect is more pronounced when the energy of the initial vibrational level is above the energy of the LICI point.

Shape and strength of dynamical couplings between vibrational levels of the H2+, HD+ and D2+ molecular ions in collision with He as a buffer gas

We present a detailed computational analysis for the interaction between the vibrating/rotating molecular ions H2+, HD+, D2+ colliding with He atoms employed as buffer gas within ion trap experiments. The production and preparation of these molecular ions from their neutrals usually generate rovibrationally excited species which will therefore require internal energy cooling down to their ground vibrational levels for further experimental handling. In this work we describe the calculation of the full 3D interaction potentials and of the ionic vibrational levels needed to obtain the vibrational coupling potential matrix elements which are needed in the multichannel treatment of the rovibrationally inelastic collision dynamics. The general features of such coupling potential terms are discussed for their employment within a quantum dynamical modeling of the relaxation processes, as well as in connection with their dependence on the initial and final vibrational levels which are directly coupled by the present potentials. As a preliminary test of the potential effects on scattering observables, we perform calculations between H2+ and He atoms at the energies of an ion-trap by using either the rigid rotor (RR) approximation or the more accurate vibrationally averaged (VA) description for the v = 0 state of the target. Both schemes are described in detail in the present paper and the differences found in the scattering results are also analysed and discussed. We further present and briefly discuss some examples of state-to-state rovibrationally inelastic cross sections, involving the two lowest vibrational levels of the H2+ molecular target ion, as obtained from our time-independent multichannel quantum scattering code.Graphical abstract

Time slicing in 3D momentum imaging of the hydrogen molecular ion photo-fragmentation.

Photo-fragmentation of the hydrogen molecular ion was investigated with 800 nm, 50 fs laser pulses by employing a time slicing 3D imaging technique that enables the simultaneous measurement of all three momentum components which are linearly related with the pixel position and slicing time. This is done for each individual product particle arriving at the detector. This mode of detection allows us to directly measure the three-dimensional fragment momentum vector distribution without having to rely on mathematical reconstruction methods, which additionally require the investigated system to be cylindrically symmetric. We experimentally reconstruct the laser-induced photo-fragmentation of the hydrogen molecular ion. In previous experiments, neutral molecules were used as a target, but in this work, performed with molecular ions, the initial vibrational level populations are well-defined after electron bombardment, which facilitates the interpretation. We show that the employed time-slicing technique allows us to register the fragment momentum distribution that reflects the initial molecular states with greater detail, revealing features that were concealed in the full time-integrated distribution on the detector.

Fourier-transform spectroscopy and deperturbation analysis of the spin-orbit coupled A(1)Σ(+) and b(3)Π states of KRb.

Fourier-transform A(1)Σ(+) - b(3)Π → X(1)Σ(+) laser-induced fluorescence spectra were recorded for the natural mixture of (39,41)K(85,87)Rb isotopologues produced in a heatpipe oven. Overall 4200 rovibronic term values of the spin-orbit coupled A(1)Σ(+) and b(3)Π states were determined with an uncertainty of about 0.01 cm(-1) in the energy range [10 850, 14 200] cm(-1) covering rotational quantum numbers J' ∈ [3, 280]. Direct deperturbation analysis of the A ∼ b complex performed within the framework of the A(1)Σ(+) ∼ b(3)ΠΩ=0,1,2 coupled-channel approach reproduced experimental data with a standard deviation of 0.004 cm(-1). Initial parameters of the internuclear potentials and spin-orbit coupling functions along with the relevant transition dipole moments were obtained by performing the quasi-relativistic electronic structure calculations. The mass-invariant molecular parameters obtained from the fit were used to predict energy and radiative properties of the A ∼ b complex for low J levels of (39)K(85)Rb as well as for (41)K(87)Rb isotopologues, allowing us to identify the most reasonable candidates for the stimulated Raman transitions between the initial uppermost vibrational levels of the a(3)Σ(+) and X(1)Σ(+) states, the intermediate levels of the A ∼ b complex, and the lowest absolute ground X(1)Σ(+)(v = 0, J = 0) state.

Collision time of a triatomic chemical reaction A + BC

The collision time is an important quantity of an elementary chemical reaction and describes the speed of the collision process in a collision reaction. In this study, we present a generalized method to calculate the collision time of a triatomic reaction in which the collision time is defined by the sum of the incoming time, the intermediate complex time, and the outgoing time. Two variables including the total distance Rtotal and Ravg, the average value of Rtotal over time, are used to compute the three components of the collision time. We compute three triatomic reactions including Ca + HCl → CaCl + H, O + HCl → OH + Cl/OCl + H, and O + HF → OH + F at different collision energies and initial diatomic vibrational levels using the quasi-classical trajectory method to confirm that the method could be reliable and reasonable. The time evolutions of Rtotal could efficiently classify the direct and indirect reactive mechanisms and reveal a distinct discrepancy of the two mechanisms. As the collision energy and initial diatomic vibrational level increase, the percentage of direct reaction trajectories increases. At the same time, the average and maximal values of collision time decrease. Comparing the maximal collision time and the reactive probability distributions of the products, it could be found that most reactive trajectories’ collision time is less than 2 ps. Moreover, the present calculations indicate that the method could be applicable to estimate the lifetime of the intermediate complex for the reaction systems with deep potential wells and the collision time of the reactions with a direct abstraction mechanism.

Investigation of electron localization in harmonic emission from asymmetric molecular ion**Project supported by the National Natural Science Foundation of China (Grant No. 11404204), the Key Project of Chinese Ministry of Education (Grant No. 211025), the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20111404120004), the Natural Science Foundation for Young Scientists of Shanxi Province, China

We theoretically investigate the electron localization around two nuclei in harmonic emission from asymmetric molecular ion. The results show that the ionization process of electron localized around one nucleus competes with its transfer process to the other nucleus. By increasing the initial vibrational level, more electrons localized around the nucleus D+ tend to transfer to the nucleus He2+ so that the ionizations of electrons localized around the nucleus He2+ increase. In this case, the difference in harmonic efficiency between HeH2+ and HeD2+ decreases while the difference in harmonic spectral structure increases. The evident minimum can be observed in the harmonic spectrum of HeH2+ compared with that in the spectral structure of HeD2+, which is due to the strong interference of multiple recombination channels originating from two nuclei. Time-dependent nuclear probability density, electron-nuclear probability density, double-well model, and time-frequency maps are presented to explain the underlying mechanisms.

Mechanism of the collision energy and reagent vibration’s effects on the collision time for the reaction Ca + HCl

The collision time which describes the speed of the collision process in a reaction is an important concept to an elementary chemical reaction. In this study, the quasiclassical trajectory method is applied to investigate the collision time of the reaction Ca+HCl (v=0–2, j=0)→CaCl+H. In order to provide a clear image of the reaction, the integral cross section we calculated is compared with corresponding quantum result and shows fairly good agreement. The results indicate that the collision energy and the initial vibrational level affect the average collision time remarkably. As the collision energy or the initial vibrational level increases, the average collision time decreases. The difference of average collision time for different initial vibrational level decreases with the increasing of collision energy. The product distributions as functions of scattering angle, attack angle and impact parameter are computed. Observing the functions, it can be found that the features could be caused by a competition among different parts of the product molecules with different collision time. For all the investigated initial vibrational levels, most of the reactive trajectories have the shorter collision times and are focused in several concentrated regions. Two possible mechanisms could be responsible for the HCl (v=0) reaction in the concentrated regions. One is the sideway scattering and the system would fall into the deep potential well once in the collision process. The other is the weak forward scattering and strong backward scattering. The system would go around the deep potential well in the collision process. It is shown that the character of the weak forward scattering and strong backward scattering for the HCl (v=1 and 2) reactions in the concentrated regions. However, the reactions outside the concentrated regions have the longer collision times and no particular mechanism. In the collision process, the system could fall into the deep potential well many times. We also explored the dynamics of the reaction at the same total energies but for different initial vibrational levels and found that the role of the insertion well becomes less and less important with the increasing of total energy.

On the impact of the temporal variability of the collisional quenching process on the mesospheric OH emission layer: a study based on SD-WACCM4 and SABER

Abstract. The mesospheric OH Meinel emissions are subject of many theoretical and observational studies devoted to this part of the atmosphere. Depending on the initial vibrational level of excitation the altitude of the considered OH Meinel emission is systematically shifted, which has important implications for the intercomparison of different studies considering different transition bands. Previous model studies suggest that these vertical shifts are essentially caused by the process of collisional quenching with atomic oxygen. Following this hypothesis, a recent study found experimental evidence of a coherent seasonality at tropical latitudes between vertical shifts of different OH Meinel bands and changes in atomic oxygen concentrations. Despite the consistent finding of the above mentioned hypothesis, it cannot be excluded that the actual temporal variability of the vertical shifts between different OH Meinel bands may in addition be controlled or even dominated by other processes. It remains an open question whether the observed temporal evolution is indeed mainly controlled by the modulation of the collisional quenching process with atomic oxygen. By means of a sensitivity study which employs a quenching model to simulations made with the SD-WACCM4 chemistry climate model, we aim at assessing this question. From this study we find that the observed seasonality of vertical OH Meinel shifts is only partially controlled by temporal changes in atomic oxygen concentrations, while molecular oxygen has another noticeable impact on the vertical OH Meinel shifts. This in particular becomes evident for the diurnal variability of vertical OH Meinel shifts, which reveal only a poor correlation with the atomic oxygen species. Furthermore, changes in the H + O3 source gases provide another mechanism that can potentially affect the diurnal variability in addition. By comparison with limb radiance observations from the SABER/TIMED satellite this provides an explanation for the less evident diurnal response between changes in O concentrations and vertical OH Meinel shifts. On the other hand, at seasonal timescales the coherency between both quantities is again evident in SABER/TIMED but less pronounced compared to our model simulations.

Theoretical Exploration of the Isotopic Effect on Molecular High-Order Harmonic Generation

ABSTRACT The isotopic effect on the generation of the molecular high-order harmonics is studied by numerically solving the one-dimensional time-dependent Schrödinger equation when the model hydrogen molecule ions/hydrogen deuterium molecule ions are exposed to an intense laser pulse. To explain the effect more clearly, not only the ionization probabilities but also the electron–nuclear probability density distributions and time-frequency profiles are calculated. The results show that more intense harmonics are generated in the asymmetric diatomic molecule ions/hydrogen deuterium molecule ions than those of hydrogen molecule ions. Moreover, the interference minimum in the harmonic spectra is investigated by adjusting the laser intensity and the initial vibrational state. It is shown that the interference minimum is sensitive to the laser intensity and the initial vibrational level for hydrogen molecule ions; in contrast, it is only dependent on the initial vibrational level for hydrogen deuterium molecule ions.

High harmonic generation in H2 + and HD + by two-colour femtosecond laser pulses

We have theoretically investigated the high harmonic generation (HHG) spectra of H2 + and HD + using a time-dependent wave packet approach for the nuclear motion with combined two-colour (1ωL–3ωL) pulsed lasers for ωL corresponding to wavelengths 1064 nm and 800 nm. The 1ωL and 3ωL lasers have peak intensities of I10 = 5.0×1013 W/cm2 and I20 = 2.0×1014 W/cm2, respectively. We have taken the pulse duration of T = 50 fs for both the fields, and the molecular initial vibrational level v0 = 0. We have argued that for these combinations, the harmonic generation due to transitions in the electronic continuum by tunnelling or multiphoton ionization may be neglected and only the electronic transitions within the two lowest electronic states would be important. Thus, the characteristic features of HHG spectra in the two-colour field are determined, in our model, by the nuclear motions on the two lowest field-coupled electronic states between which interelectronic and intraelectronic (due to the intrinsic dipole moments in case of HD + ) radiative transitions can take place. We have studied the role of relative phase (φ) of the two fields on the HHG spectra of the molecular ions. In case of HD + , the effect of nonadiabatic (NA) nonradiative interaction between the two lowest Born-Oppenheimer (BO) electronic states (1sσg, 2pσu) has been taken into account. Our calculations give realistic HHG spectra which are reasonably efficient and extended for both H2 + and HD + in the mixed two-colour field without involving the electronic continuum. The use of two-colour (1ωL–3ωL) field enables us to generate high harmonics beyond that achievable with a single 1ωL or 3ωL field of the corresponding intensity, frequency and pulse time.