L. Rubio-Lago

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.

A slice imaging and multisurface wave packet study of the photodissociation of CH3I at 304 nm.

The role of the conical intersection between the (1)Q(1) and (3)Q(0) excited states in the photodissociation of CH(3)I at 304 nm is investigated drawing a comparison between the adiabatic--through direct absorption to the (3)Q(1) state--and non-adiabatic--via the (1)Q(1)→(3)Q(0) conical intersection--production of I atoms in the ground (2)P(3/2) state. The versatility of the slice imaging technique in combination with resonance enhanced multiphoton ionization (REMPI) detection of I((2)P(3/2)) atoms allow distinct measurements of the competing processes. The I((2)P(3/2)) atom kinetic energy distributions (KEDs) obtained in both cases reflect inverted vibrational progressions of the ν(2) umbrella mode of the CH(3) co-product. The experimental results show a satisfactory agreement with multisurface wave packet calculations using a reduced dimensionality (pseudotriatomic) model carried out on the available ab initio potential energy surfaces.

Communication: First observation of ground state I(2P3/2) atoms from the CH3I photodissociation in the B-band

The photodissociation of CH(3)I in the second absorption band (the B-band) has been studied at the wavelength 199.11 nm, coincident with the 3(0)(1) (3)R(1)(E)←X((1)A(1)) CH(3)I vibronic transition, using a combination of slice imaging and resonance enhanced multiphoton ionization detection of the CH(3) fragment. The kinetic energy and angular distributions of the recoiling CH(3) fragment confirm a major predissociation dynamics channel as a result of the interaction between the bound (3)R(1) Rydberg state and the repulsive (3)A(1)(E) state--ascribed to the A-band--yielding CH(3) fragments in correlation with spin-orbit excited I*((2)P(1/2)) atoms. In addition, first evidence of a non-negligible population of ground state I((2)P(3/2)) atoms in the CH(3) fragment slice images, suggests a secondary predissociation mechanism via interaction between the (3)R(1) Rydberg state and the repulsive A-band (1)Q(1) state.

Photodissociation of pyrrole-ammonia clusters below 218 nm: quenching of statistical decomposition pathways under clustering conditions.

The excited state hydrogen transfer (ESHT) reaction in pyrrole-ammonia clusters (PyH·(NH(3))(n), n = 2-5) at excitation wavelengths below 218 nm down to 199 nm, has been studied using a combination of velocity map imaging and non-resonant detection of the NH(4)(NH(3))(n-1) products. Special care has been taken to avoid evaporation of solvent molecules from the excited clusters by controlling the intensity of both the excitation and probing lasers. The high resolution translational energy distributions obtained are analyzed on the base of an impulsive mechanism for the hydrogen transfer, which mimics the direct N-H bond dissociation of the bare pyrrole. In spite of the low dissociation wavelengths attained (~200 nm) no evidence of hydrogen-loss statistical dynamics has been observed. The effects of clustering of pyrrole with ammonia molecules on the possible statistical decomposition channels of the bare pyrrole are discussed.

A velocity-map imaging study of methyl non-resonant multiphoton ionization from the photodissociation of CH3I in the A-band

Chemical reaction dynamics and, particularly, photodissociation in the gas phase are generally studied using pump–probe schemes where a first laser pulse induces the process under study and a second one detects the produced fragments. Providing an efficient detection of ro-vibrationally state-selected photofragments, the resonance enhanced multiphoton ionization (REMPI) technique is, without question, the most popular approach used for the probe step, while non-resonant multiphoton ionization (NRMPI) detection of the products is scarce. The main goal of this work is to test the sensitivity of the NRMPI technique to fragment vibrational distributions arising from molecular photodissociation processes. We revisit the well-known process of methyl iodide photodissociation in the A-band at around 280 nm, using the velocity-map imaging technique in conjunction with NRMPI of the methyl fragment. The detection wavelength, carefully selected to avoid any REMPI transition, was scanned between 325 and 335 nm seeking correlations between the different observables—the product vibrational, translational and angular distributions—and the excitation wavelength of the probe laser pulse. The experimental results have been discussed on the base of quantum dynamics calculations of photofragment vibrational populations carried out on available ab initio potential-energy surfaces using a four-dimensional model. This article is part of the themed issue ‘Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces’.

Femtosecond Photodissociation Dynamics by Velocity Map Imaging. The Methyl Iodide Case

The introduction of time-resolved measurements in the femtosecond time-scale using velocity map imaging techniques of charged particles (ions and photoelectrons) in combination with resonant multiphoton ionization of the fragments for the study of the photodissociation dynamics of small polyatomic molecules is reviewed. A typical experiment consists of the measurement of a sequence of images, whose analysis requires in most cases sophisticated multidimensional fitting methods to extract all the relevant time-resolved information contained in the images. In particular, the application of these techniques to the study of the direct photodissociation (A band) and electronic predissociation (B band) of methyl iodide along with the detection and characterization of transient species and the study of cluster dissociation, as a case example for femtosecond velocity map imaging, are presented and discussed.