Experimental Research

Molecular Photodissociation Dynamics by Ion Imaging Techniques and Nanosecond Laser Pulses

Time-resolved Molecular Photodissociation Dynamics and Control by Ion and Photoelectron Imaging Techniques and Femtosecond Laser Pulses

Molecular Alignment Dynamics and Control

Nanofoaming in biopolymers by femtosecond laser pulses

Laser ablation, Pulsed Laser Deposition (PLD), and Matrix Assisted Laser Desorption/Ionization (MALDI)

Molecular Photodissociation Dynamics by Ion Imaging Techniques and Nanosecond Laser Pulses

 In our lab we have implemented the velocity map and slice ion imaging techniques to study the dynamics of photodissociation processes in the gas phase.

 The velocity map ion imaging (VMI) technique, sketched in the picture below, was first developed by Eppink and Parker in 1997 [Rev. Sci. Instrum. 68, 3477 (1997)] and since then it has been a basic tool in experimental studies of the dynamics of chemical reactions in many laboratories around the world. The VMI technique consists in projecting the 3D photofragment ion distribution into a 2D position sensitive detector attached to a phosphor screen and CCD camera. The use of the VMI technique requires cylindrical symmetry in the experiment in order to perform the Abel inversion mathemzatical procedure to recover the central slice of the 3D distribution from the measured 2D image.

 In order to overcome this inconvenient, slice imaging techniques, and, in particular, the pulsed slice imaging technique was first demonstrated by Kitsopoulos and co-workers in 2001 [C. R. Gebhardt, T. N. Kitsopoulos, et al., Rev. Sci. Instrum. 72, 3848 (2001)] and later in 2006 using a single electric field configuration [V. Papadakis and T. N. Kitsopoulos, Rev. Sci. Instrum. 77, 083101 (2006)]. By using a delayed pulsed extraction electric field the 3D ion cloud is allowed to grow enough as to being able to gate the central slice just at the detector. In this way, no Abel inversion is needed and the measured ion image can be analyzed directly.

 During the last years we have studied molecular photodissociation dynamics and stereodynamics using velocity map and slice imaging techniques in combination with nanosecond laser pulses and in the following we will briefly summarize the main achievements.



Photodissociation of CH3SH in the first and second absorption bands

 The CH3(X 2A1) + SH(X 2Π) channel of the photodissociation of CH3SH has been investigated at several wavelengths in the first 1 1A"X 1A' and second 2 1A"X 1A' absorption bands by means of velocity map imaging of the CH3 fragment. A fast highly anisotropic (β = -1 ± 0.1) CH3( X 2A1) signal has been observed in the images at all the photolysis wavelengths studied, which is consistent with a direct dissociation process from an electronically excited state by cleavage of the C-S bond in the parent molecule. From the analysis of the CH3 images, vibrational populations of the SH(X 2Π) counter-fragment have been extracted. In the second absorption band, the SH fragment is formed with an inverted vibrational distribution as a consequence of the forces acting in the crossing from the bound 2 1A" second excited state to the unbound 1 1A" first excited state. The internal energy of the SH radical increases as the photolysis wavelength decreases. In the case of photodissociation via the first excited state, the direct production of CH3 leaves the SH counter fragment with little internal excitation. Moreover, at the longer photolysis wavelengths corresponding to excitation to the 1 1A" state, a slower anisotropic CH3 channel has been observed (β = -0.8 ± 0.1) consistent with a two step photodissociation process, where the first step corresponds to the production of CH3S(X 2E) radicals via cleavage of the S-H bond in CH3SH, followed by photodissociation of the nascent CH3S radicals yielding CH3(X 2A1) + S(X 3P0,1,2).

[G. A. Amaral, F. Ausfelder, J. G. Izquierdo, L. Rubio-Lago, L. Bañares, Imaging the photodissociation of CH3SH in the first and second absorption bands. The CH3(X 2A1) + SH(X 2Π) channel, J. Chem. Phys., 126, 024301 (2007)]

Photodissociation of CH3CHO by velocity map and slice imaging. The radical and molecular channels

 The photodissociation of acetaldehyde in the molecular channel yielding CO and CH4 at 248 nm has been studied probing different rotational states of the CO(v=0) fragment by slice ion imaging using a 2+1 REMPI scheme at around 230 nm. From the slice images, a clear evidence of the co-existence of two different mechanisms has been obtained. One of the mechanisms is consistent with the well studied conventional transition state in which CO products appear rotationally excited, and the second is consistent with a roaming mechanism. This roaming mechanism is characterized by a low rotational energy disposal into the CO fragment as well as by a very low kinetic energy release, corresponding with high internal energy in the CH4 counter fragment. This research has been carried out in collaboration with Theofanis Kitsopoulos (FORTH Crete, Greece).

[L. Rubio-Lago, G. A. Amaral, A. Arregui, J. G. Izquierdo, F. Wang, D. Zouris, T. N. Kitsopoulos, L. Bañares, Slice imaging of the photodissociation of acetaldehyde at 248 nm. Evidence of a roaming mechanism, Phys. Chem. Chem. Phys., 9, 6123 (2007)]

 The photodissociation of acetaldehyde in the radical channel yielding CH3 and HCO has been studied at wavelengths between 315 and 325 nm using the VMI technique. Upon one-photon absorption at 315 nm, the molecule is excited to the first singlet excited state S1, which, in turn, undergoes intersystem crossing to the first excited triplet state T1. On the triplet surface, the molecule dissociates into CH3 and HCO radicals with large kinetic energy release (KER), in accordance with the well characterized exit barrier on T1. However, at longer wavelengths (>320 nm), which correspond to excitation energies just below the triplet barrier, a sudden change in KER is observed. At these photolysis wavelengths, there is not enough energy to surpass the exit barrier on the triplet state, which leaves the possibility of unimolecular dissociation on S0 after internal conversion from S1. We have characterized the fragments’ KER at these wavelengths, as well as determined the energy partitioning for the radical fragments. A new accurate estimate of the barrier height on T1 has been obtained.

[G. A. Amaral, A. Arregui, L. Rubio-Lago, J. D. Rodríguez, L. Bañares, Imaging the radical channel in acetaldehyde photodissociation: Competing mechanisms at energies close to the triplet exit barrier, J. Chem. Phys., 133, 064303 (2010)]

 The roaming dynamics in the photodissociation of acetaldehyde was studied through the first absorption band, in the wavelength interval ranging from 230 nm to 325 nm. Using a combination of the velocity-map imaging technique and rotational resonance enhanced multiphoton ionization (REMPI) spectroscopy of the CO fragment, the branching ratio between the canonical transition state and roaming dissociation mechanisms was obtained at each of the photolysis wavelengths studied. Upon one photon absorption, the molecule is excited to the first singlet excited S1 state, which, depending on the excitation wavelength, either converts back to highly vibrationally excited ground S0 state or undergoes intersystem crossing to the first excited triplet T1 state, from where the molecule can dissociate over two main channels: the radical (CH3+HCO) and the molecular (CO+CH4) channels. Three dynamical regions are characterized: in the red edge of the absorption band, at excitation energies below the T1 barrier, the ratio of the roaming dissociation channel increases, largely surpassing the transition state contribution. As the excitation wavelength is increased, the roaming propensity decreases reaching a minimum at wavelengths ≈308 nm. Towards the blue edge, at 230 nm, an upper limit of ≈50% has been estimated for the contribution of the roaming channel. The experimental results are interpreted in terms of the interaction between the different potential energy surfaces involved by means of ab initio stationary points and intrinsic reaction coordinate paths calculations.

[L. Rubio-Lago, G. A. Amaral, A. Arregui, J. González-Vázquez, L. Bañares, Imaging the molecular channel in acetaldehyde photodissociation: roaming and transition state mechanisms, Phys. Chem. Chem. Phys., 14, 6067 (2012)]

Photodissocation of CH3I in the red and blue edges of the A band

 The photodissociation of methyl iodide at different wavelengths in the red edge of the A-band (286-333 nm) has been studied using a combination of slice imaging and resonance enhanced multiphoton ionization detection of the methyl fragment in the vibrational ground state (ν=0). The kinetic energy distributions (KED) of the produced CH3(ν=0) fragments show a vibrational structure, both in the I(2P3/2) and I*(2P1/2) channels, due to the contribution to the overall process of initial vibrational excitation in the ν3(C-I) mode of the parent CH3I. The structures observed in the KEDs shift toward upper vibrational excited levels of CH3I when the photolysis wavelength is increased. The I(2P3/2)/I*(2P1/2) branching ratios, photofragment anisotropies, and the contribution of vibrational excitation of the parent CH3I are explained in terms of the contribution of the three excited surfaces involved in the photodissociation process, 3Q0, 1Q1, and 3Q1, as well as the probability of nonadiabatic curve crossing 1Q13Q0. The experimental results are compared with multisurface wave packet calculations carried out using the available ab initio potential energy surfaces, transition moments, and nonadiabatic couplings, employing a reduced dimensionality (pseudotriatomic) model. A general qualitative good agreement has been found between theory and experiment, the most important discrepancies being in the Φ=[I]/([I]+[I*]) quantum yield. Inaccuracies of the available potential energy surfaces are the main reason for the discrepancies. This research has been carried out in collaboration with Alberto García-Vela (IFF, CSIC, Madrid).

[L. Rubio-Lago, A. García-Vela, A. Arregui, G. A. Amaral, L. Bañares, The photodissociation of CH3I in the red edge of the A-band: Comparison between slice imaging experiments and multisurface wave packet calculations, J. Chem. Phys., 131, 174309 (2009)]

 The photodissociation of CH3I 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 CH3 fragment in the vibrational ground state (ν=0). The profiles of the CH3(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 3Q0 and 1Q1 states. In the studied region, unlike in the central part of the A-band where absorption to the 3Q0 state dominates, the I(2PJ), with J=1/2,3/2, in correlation with CH3(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 3Q01Q1 non-adiabatic dynamics at the bluest excitation wavelengths both in the fragment kinetic energy and angular distributions.

[M. G. González, J. D. Rodríguez, L. Rubio-Lago, A. García;-Vela and L. Bañares, Slice imaging and wave packet study of the photodissociation of CH3I in the blue edge of the A-band: evidence of reverse 3Q01Q1 non-adiabatic dynamics, Phys. Chem. Chem. Phys., 13, 16404 (2011)]

Time-resolved Molecular Photodissociation Dynamics and Control by Ion and Photoelectron Imaging Techniques and Femtosecond Laser Pulses

Femtosecond real time photodissociation dynamics of CH3I from the A band

 Breaking and forming of chemical bonds and energy flow in molecules are at the very heart of Chemistry. The experimental observation of chemical transformations in real time is the focus of Femtochemistry, which utilizes ultrashort laser pulses to diagnose the time evolution of a chemical reaction. Almost two decades after the birth of this new field in Chemistry and with a wealth of accumulated information concerning molecular dynamics even in very complex, often biologically relevant systems, many basic questions about the real time dynamics of simple systems remain still unresolved, and it is only via the combination of recently developed detection techniques and the full exploitation of state-of-the-art femtosecond laser systems, that we can hope to address them. We have demonstrated that the combination of velocity map imaging and femtosecond pump-probe technology using resonant detection of ground-state neutral fragments allows the measurement of the reaction times of several well-defined channels in the bond breakage of a polyatomic molecule. We have been able to clock the C-I bond rupture of methyl iodide, CH3I, from the A-band, which involves non-adiabatic dynamics yielding ground-state I(2P3/2) and spin-orbit excited I*(2P1/2) and ground and vibrationally excited CH3 fragments. The reaction times found for the different channels are directly related with the non-adiabatic dynamics of this multidimensional photodissociation reaction. The present experiment paves the way to time-resolved and channel-selected studies of fragmentation dynamics in complex molecules.

[R. de Nalda, J. G. Izquierdo, J. Durá, L. Bañares, Femtosecond multichannel photodissociation dynamics of CH3I from the A band by velocity map imaging, J. Chem. Phys., 126, 021101 (2007)]

Molecular alignment dynamics and control

Pulse shaping control of alignment dynamics in N2

 Control on the alignment transients of impulsively aligned ensembles of N2 molecules has been demonstrated by the use of laser pulses shaped by a spatial light modulator. An alignment experiment has been inserted in the feedback loop of an evolutionary algorithm that found optimum pulse shapes for a set of criteria. Optimum pulse shapes for the maximization of total alignment and for the control of certain aspects of the revival structures are given. The physical mechanisms responsible for the control are analysed with the help of single-parameter control schemes and numerical simulations, which allowed us to explore the low-temperature region. This approach sheds light on the role played by different control mechanisms for the alignment dynamics of a molecular ensemble.

 These experiments have been carried out in collaboration with the group of Prof. Thomas Baumert (University of Kassel, Germany) with financial support through German-Spanish ''Acciones Integradas'' project.


[C. Horn, M. Wollenhaupt, M. Krug, T. Baumert, R. de Nalda, L. Bañares, Adaptive control of molecular alignment, Phys. Rev. A, 73, 031401(R) (2006); R. de Nalda, C. Horn, M. Wollenhaupt, M. Krug, L. Bañares, T. Baumert, Pulse shaping control of alignment dynamics in N2, J. Raman Spect., 38, 543 (2007)]

Nanofoaming in biopolymers by femtosecond laser pulses

 Nanostructuring induced in femtosecond laser irradiation of biopolymers is examined in self-standing films of collagen and gelatine. Irradiation by single 90 fs pulses at 800, 400 and 266 nm is shown to result in the formation of a modified layer with submicrometric size structures. The size and uniformity of the observed features are strongly dependent on irradiation wavelength and on the characteristics of the biopolymer (water content and mechanical strength). Examination of the films by laser induced fluorescence serves to assess the chemical modifications induced by laser irradiation, revealing changes in the emission bands assigned to the aromatic amino acid tyrosine and its degradation products. The results are discussed in the framework of a mechanism involving the generation of large free-electron densities, through multiphoton and avalanche ionization, which determine the temperature and stress distribution in the irradiated volume.

 This research line is carried out in collaboration with Marta Castillejo and Rebeca de Nalda from Rocasolano Institute of Physical Chemistry, CSIC, Madrid, Spain.

[S. Gaspard, M. Oujja, R. de Nalda, C. Abrusci, F. Catalina, L. Bañares, S. Lazare, M. Castillejo, Nanofoaming in biopolymers by femtosecond pulsed laser irradiation, App. Surf. Sci., 254, 1179 (2007)]

Laser ablation, Pulsed Laser Deposition (PLD), and Matrix Assisted Laser Desorption/Ionization (MALDI)

Solvent-free MALDI investigation of the cationization of polyethers with alkali metals

 The MALDI technique with solvent-free sample preparation has been applied to evaluate relative gas-phase affinities of polyether chain polymers with alkali metal cations. The study is perfomed on poly(ethylene glycol) and poly(propylene glycol) polymers of different lengths (PEG600, PEG1000, PPG 425, PPG 750) and the alkali metal cations Li+, Na+, K+, Cs+. The experiments show that the lattice energy of the alkali salts employed as cation precursor can have a strong influence on the outcome of conventional MALDI measurements. With the solvent-free method, these crystal binding effects can be made negligible by combining in the same sample alkali salts with different counterions. The recorded MALDI spectra show that the polyether-cation aggregation efficiencies decrease systematically with growing cation size. This cation size selectivity is considerably enhanced for the polymers with the shorter chains, which can be attributed to the reduced ability of the polymer to build a coordination shell around the larger cations. The steric effects introduced by the side CH3 group of propylene glycol with respect to ethylene glycol also enhance the preference for cationization of the polymer by the smaller cations. These observations correct some qualitative trends derived from previous studies, which did not account for lattice energy effects of the cation precursors.

 This work has been carried out in collaboration with Prof. Bruno Martínez Haya from Universidad Pablo de Olavide (Seville, Spain).

[A. R. Hortal, P. Hurtado, B. Martínez-Haya, A. Arregui, L. Bañares, Solvent-free MALDI investigation of the cationization of polyethers with alkali metals, J. Phys. Chem. B 112, 8530 (2008)]


Laser ablation of CdS with nanosecond laser pulses

 The formation of cationic clusters in the laser ablation of CdS targets has been investigated as a function of wavelength and fluence by mass spectrometric analysis of the plume. Ablation was carried out at the laser wavelengths of 1064, 532, 355 and 266 nm in order to scan the interaction regimes below and above the energy band gap of the material. In all cases, the mass spectra showed stoichiometric CdnSn+ and non stoichiometric CdnSn-1+, CdnSn+1+ and CdnSn+2+ clusters up to 4900 amu. Cluster size distributions were well represented by a log-normal function, although larger relative abundance for clusters with n=13, 16, 19, 34 was observed (magic numbers). The laser threshold fluence for cluster observation was strongly dependent on wavelength, ranging from around 16 mJ/cm2 at 266 nm to more than 300 mJ/cm2 at 532 and 1064 nm. According to the behavior of the detected species as a function of fluence, two distinct families were identified: the “light” family containing S2+ and Cd+, and the ''heavy'' clusterized family grouping Cd2+ and CdnSm+. In terms of fluence, it has been determined that the best ratio for clusterization is achieved close to the threshold of appearance of clusters at all wavelengths. At 1064, 532, and 355 nm, the production of ''heavy'' cations as a function of fluence showed a maximum, indicating the participation of competitive effects, whereas saturation is observed at 266 nm. In terms of relative production, the contribution of the ''heavy'' family to the total cation signal was significantly lower for 266 nm than for the longer wavelengths. Irradiation at 355 nm in the fluence region of 200 mJ/cm2 has been identified as the optimum for the generation of large clusters in CdS.

 This work has been carried out in collaboration with Drs. Margarita Martín, Rebeca de Nalda and Jesús Álvarez from Rocasolano Institute of Physical Chemistry, CSIC, Madrid, Spain

[J. Álvarez, M. López, R. de Nalda, M. Martín, A. Arregui, L. Bañares, Generation of clusters from CdS laser ablation: the role of wavelength and fluence, Appl. Phys. A, 95, 681 (2009)]
Pulsed laser deposition of TiO2 and CdS

 Pulsed Laser Deposition (PLD) using femtosecond (fs) laser pulses is an advantageous and efficient technique for the production of nanoparticles (NP) with uniform and narrow size distributions due to the extreme temperature and pressure conditions generated in the pulse target interaction. Fs–PLD has been used as a route to generate NP of several metals and semiconductors. TiO2 is a wide-band semiconductor with exceptional optical and electronic properties with many applications in photovoltaic devices, sensors, optical coatings and as photocatalyst. The performance in these applications can be enhanced when using nanostructured material. In this work we have concentrated in the characterization of the nanostructured deposits grown on Si (100) substrate produced by irradiating a TiO2 sintered target in vacuum or in oxygen using a Ti:sapphire laser delivering 800 nm, 90-250 fs pulses. The influence of laser fluence, repetition frequency, pulse duration, temperature of substrates and oxygen gas pressure was studied. The stoichiometry of the deposits was determined by X-ray photoelectron spectroscopy (XPS) and their surface morphology observed by Environmental Scanning Electron Microscopy (ESEM) and atomic force microscopy (AFM). The deposited films, with the same chemical composition as the target, consist in a distribution of NPs with diameters in the range of 10-200 nm. It is observed that deposition under oxygen favours the production of small size NPs, while heating the substrate ensures a higher density of particulates. Comparison with previous results obtained by PLD using a nanosecond pulsed laser shows the advantage of fs pulses to produce nanostructured deposits.

 This research line is carried out in collaboration with Marta Castillejo, Rebeca de Nalda and Mikel Sanz from Rocasolano Institute of Physical Chemistry, CSIC, Madrid, Spain.

[M. Sanz, M. Walczak, R. de Nalda, M. Oujja, J. F. Marco, J. Rodriguez, J. G. Izquierdo, L. Bañares, M. Castillejo, Femtosecond pulsed laser deposition of nanostructured TiO2 films, App. Surf. Sci., 255, 5206 (2009)]

Chemical reaction dynamics and femtochemistry