Photodissociation dynamics of methylamine in the blue edge of the A-band. II. The NH2 + CH3 channel

The photodissociation dynamics leading to the C-N bond cleavage in methylamine (CH3NH2) are investigated upon photoexcitation in the blue edge of the first absorption A-band, in the 198-204 nm range. Velocity map images of the generated methyl (CH3) fragment detected in specific vibrational modes, i.e., ν = 0, ν1 = 1, and ν2 = 1, through resonance enhanced multiphoton ionization, are presented along with the corresponding translational energy distributions and the angular analysis. The experimental results are complemented by high-level ab initio calculations of potential energy curves as a function of the C-N bond distance. While a similar single Boltzmann-type contribution is observed in all the translational energy distributions measured, the speed-dependent anisotropy parameter obtained through the angular analysis reveals the presence of two different mechanisms. Prompt dissociation through the conical intersection between the Ã1A' first excited state and the ground state located in the exit channel is, indeed, revealed as a minor channel. In contrast, slow dissociation on the ground state, presumably from frustrated N-H bond cleavage trajectories, constitutes the major reaction pathway leading to the methyl formation.

Formation of highly excited iodine atoms from multiphoton excitation of CH3I

Mass resolved REMPI spectra, as well as CH₃⁺and I⁺ ion and photoelectron images, were recorded for two-photon resonant excitations of CH₃I via s, p and d Rydberg states (CH₃I**) in the excitation region of 55 700 to 70 000 cm^-1. Photoelectron (PE) and ion kinetic energy release spectra (KERs) were derived from the images. The data revealed that after the two-photon resonant excitation, an additional photon is absorbed to form one or more superexcited state(s) (CH₃I^#), followed by branching into three pathways. The major one is the dissociation of CH₃I^# to form excited Rydberg states of iodine atoms (I**) along with CH₃(X), a phenomenon not commonly observed in methyl halides. The second (minor) pathway involves autoionization of CH₃I^# towards CH₃I⁺(X), which absorbs another photon to form CH₃⁺ along with I/I* and the third one (minor) is CH₃I^# dissociation towards the ion pair, CH₃⁺ + I^-, prior to I^- electron ejection. Furthermore, one-photon non-resonant dissociation of CH₃I to form CH₃(X) and I/I* prior to three-photon ionization of the fragments is also detected.

Photodissociation dynamics of bromoiodomethane from the first and second absorption bands. A combined velocity map and slice imaging study

Ion imaging is applied to disentangle the selective bond cleavage in the photodissociation of bromoiodomethane from the two first absorption bands.

Angular distributions for the inelastic scattering of NO(X2Π) with O2(X3Σg-)

The inelastic scattering of NO(X²Π) by O₂(X³Σ_g^-) was studied at a mean collision energy of 550 cm^-1 using velocity-map ion imaging. The initial quantum state of the NO(X²Π, v = 0, j = 0.5, Ω=0.5, 𝜖 = -1, f) molecule was selected using a hexapole electric field, and specific Λ-doublet levels of scattered NO were probed using (1+1^') resonantly enhanced multiphoton ionization. A modified "onion-peeling" algorithm was employed to extract angular scattering information from the series of "pancaked," nested Newton spheres arising as a consequence of the rotational excitation of the molecular oxygen collision partner. The extracted differential cross sections for NO(X) f→f and f→e Λ-doublet resolved, spin-orbit conserving transitions, partially resolved in the oxygen co-product rotational quantum state, are reported, along with O₂ fragment pair-correlated rotational state population. The inelastic scattering of NO with O₂ is shown to share many similarities with the scattering of NO(X) with the rare gases. However, subtle differences in the angular distributions between the two collision partners are observed.

An improved spin-orbit coupling model for use within the effective relativistic coupling by asymptotic representation (ERCAR) method

An improved atomic spin-orbit model is presented, which is designed to be used within the framework of the effective relativistic coupling by asymptotic representation method. This method is used for the generation of highly accurate coupled potential energy surfaces (PESs) to represent the fine structure energies of appropriate systems. The approach is demonstrated using CH₃I and its photodissociation as a typical example. The method is based on a specific diabatization of electronic spin-space ("spin-free") states with respect to the asymptote at which a single relativistic atom is separated from a molecular non-relativistic fragment. Thus, the relativistic coupling effects can be treated entirely within the atomic framework. So far, an effective spin-orbit coupling operator which only accounts for intra-state coupling within each atomic spin-space state was used. In the present work, this approach is extended to account for inter-state couplings among different atomic spin-space states as well. It is shown that this extended approach improves the accuracy of the PESs significantly for higher excited states and also enhances the accuracy of low energy states. In particular, it improves the representation of the spin-orbit induced conical intersection among the ³Q₀ and ¹Q₁ states of CH₃I, which is of high relevance for the nonadiabatic quantum dynamics of the photodissociation.

Photodissociation dynamics of ICH2Cl → CH2Cl + I*/I: photofragment translational spectroscopy at 304 and 277 nm

The photodissociation dynamics of ICH2Cl → CH2Cl + I*/I at 304 and 277 nm has been investigated with our mini-TOF photofragment translational spectrometer with a weak acceleration field of <1 V cm(-1). Many peaks are resolved or partially resolved in the TOF spectra and the photofragment translational spectra (PTS) of both the I*((2)P1/2) channel and the I((2)P3/2) channel. These resolved peaks are assigned to the C-Cl stretch vibrational states of the CH2Cl fragment. The rotational energy ER of the CH2Cl fragment is highly excited due to its asymmetric structure. The value of ER/ET is measured to be about 0.71. In the I* channel, the partitioning of the available energy Eavl into the translational energy ET, the rotational energy ER, and the vibrational energy EV for each resolved vibrational state has been calculated.

New insights into the photodissociation of methyl iodide at 193 nm: stereodynamics and product branching ratios

Stereodynamics imaging provides new insights into the photodissociation of methyl iodide at 193 nm.

Imaging the stereodynamics of methyl iodide photodissociation in the second absorption band: fragment polarization and the interplay between direct and predissociation

Stereodynamics imaging disentangles the interplay between direct and predissociation in the onset of the second absorption band of methyl iodide.

Spectroscopic study of the I2 formation from the photolysis of iodomethanes (CHI3, CH2I2, CH3I, and CH2ICl) at different wavelengths

Emission spectra following the photolysis of iodomethanes (CHI3, CH2I2, CH3I, and CH2ICl) at 266 nm were recorded in a slow flow cell. In addition to emission from the electronically excited species including CH (A(2)Δ, B(2)Σ(-), and C(2)Σ(+)), C2 (d(3)Πg), and atomic iodine ((4)P(o)), a series of emission bands was observed in the 12,000-19,000 cm(-1) region. The dominant structure of these emission bands was verified as the I2 B(3)Π(+)(0,u)-X(1)Σ(+)g emission at the 532 nm excitation, and the observed I2 was formed from collisions between iodine atoms generated from the C-I bond dissociation in these iodomethanes. The I2 emission spectra following the photolysis of CH2I2 at different wavelengths were acquired, and the threshold energy for the first C-I bond cleavage was determined to be 208 ± 1 kJ mol(-1). We also obtained the emission spectra of pure I2 at several visible excitation wavelengths for comparison with those from the photolysis of iodomethanes, and a least-squares global fit of the observed I2 emission bands yields more accurate anharmonicity parameters for the vibrational structure in the I2 B-X transition.

Slice imaging and wave packet study of the photodissociation of CH3I in the blue edge of the A-band: evidence of reverse 3Q0 ← 1Q1 non-adiabatic dynamics

The photodissociation of CH(3)I in the blue edge (217-230 nm) of the A-band has been studied using a combination of slice imaging and resonance enhanced multiphoton ionization (REMPI) detection of the CH(3) fragment in the vibrational ground state (ν = 0). The profiles of the CH(3) (ν = 0) kinetic energy distributions and the photofragment anisotropies are interpreted in terms of the contribution of the excited surfaces involved in the photodissociation process, as well as the probability of non-adiabatic curve crossing between the (3)Q(0) and (1)Q(1) states. In the studied region, unlike in the central part of the A-band where absorption to the (3)Q(0) state dominates, the I((2)P(J)), with J = 1/2, 3/2, in correlation with CH(3) (ν = 0) kinetic energy distributions show clearly two contributions of different anisotropy, signature of the competing adiabatic and non-adiabatic dynamics, whose ratio strongly depends on the photolysis wavelength. The experimental results are compared with multisurface wave packet calculations carried out using the available ab initio potential energy surfaces, transition moments, and non-adiabatic couplings, employing a reduced dimensionality model. A good qualitative agreement is found between experiment and theory and both show evidence of reverse (3)Q(0)←(1)Q(1) non-adiabatic dynamics at the bluest excitation wavelengths both in the fragment kinetic energy and angular distributions.