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Revista mexicana de física

Print version ISSN 0035-001X

Rev. mex. fis. vol.62 n.3 México May./Jun. 2016

 

Research

Dissociation-ionization and ionization-dissociation by multiphoton absorption of acetaldehyde at 266 and 355 nm. Dissociation pathways

J.C. Povedaa 

I. Álvarezb 

A. Guerrero-Tapiab 

C. Cisnerosb 

aLaboratorio de Espectroscopia Atómica Molecular, Escuela de Química-Facultad de Ciencias-Universidad Industrial de Santander, Bucaramanga Santander-Colombia A.A. 678. e-mail: jcpoveda@uis.edu.co; jkclim@fis.unam.mx

bLaboratorio de Colisiones Atómicas, Moleculares y Óptica Experimental, Instituto de Ciencias Físicas-Universidad Nacional Autónoma de México, Cuernavaca-Morelos México, 62210.

Abstract

The experimental results from the interaction of a sample of acetaldehyde (CH3CHO) with laser radiation at intensities between 109 and 1011Wcm2 and wavelengths of 266 and 355 nm are reported. As a result of multiple photon absorption, cations from ionization-dissociation (I-D) or dissociation-ionization (D-I) processes, were detected using a reflectron time of flight mass spectrometer. The processes I-D is predominant at 355 nm and D-I is predominant at 266 nm. The formation of different ions is discussed. From analysis of the ratios between the ion currents [I( CH3 CO+)+I( CO+)]/[ I( CH3+)+I( HCO+)], originated from the C-C bond or from the C-H bond breaking at different laser intensities, the predominant channels are determined.

Keywords: Acetaldehyde; photoionization; photodissociation; multiphoton processes; molecular dissociation

PACS: 32.80.Rm; 32.80.Wr; 33.15.Ta

1.Introduction

Different sources contribute to the formation of acetaldehyde: anthropogenic, plants, biomass burning, surface of the oceans, being its largest source, the oxidation of hydrocarbons. Photochemistry of acetaldehyde had been previously analyzed; it can be decomposed by photolysis 1. The interaction of acetaldehyde with the radical HO allows the hydrogen abstraction to produce the acetyl radical, and with ions and radicals such as HO and NO2 produces peroxyacetyl nitrate (CH3C(O)O2NO2). Acetaldehyde has been detected during combustion processes as a part of radical reactions 2, is sensible to oxidation processes, resulting in the formation of acetic acid 3. It also has been studied using different experimental and theoretical methods4,5,6,7,8,9. The electronic, rotational and vibrational structure has been reported10,11,12,13,14. Electronic energy levels and associated vibrational progressions have been measured using various techniques 15,16. Due to the fact that acetaldehyde is a small molecule, the analysis of experimental data can be accomplished comparing them with quantum chemical calculations15,16,17,18. Previous results show that when the acetaldehyde molecule absorbs photons in the UV wavelength range, between 250 and 350 nm 19 the transition S0 → S1 occurs through a nπ* states and it dissociates through various channels that lead to different neutral products. In condensed and gas phases vinyl alcohol can be formed by intermolecular proton transfer keto-enol equilibrium process 20,21,22,23:

CH3CHOCH2=C(H)OH 1

The energies of the first excited electronic states of acetaldehyde are shown in Table I.

Table 1 Excitation energies of acetaldehyde. 

a Time-Dependent Density Functional Theory, Functionasl B3LYP and B3P86, 6-311++G(3df,3pd) basis set.

b Time-Dependent Density Functional Theory, Functional B3P86, 6-311++G(d; p) basis set.

c Equation-of-Motion Coupled Clusters Method, [5s3d2d/3s2p] basis set.

The accessible dissociation channels by absorption of a single photon at wavelengths below 318 nm (3.89 eV) are:

CH3CHOCH3+HCO 2

CH3CHOCH3CO+H 3

The channel (2) is possible through the T1 state by an intersystem crossing process (ISC) with the S1state 24,1. The radicals CH3 and HCO can result from the dissociation of the acetyl radical after hydrogen abstraction 25,26. In the S1 state, the acetaldehyde can dissociate to form hydrogen and the CH3CO radical in an excited state, following the channel (3). Furthermore, as it has been reported 27, CH4 and CO must arise from the non-degenerate singlet ground state S0, channel (4),

CH3CHOCH4+CO 4

Other important channel is the hydrogen elimination channel (5):

CH3CHOCH2CHO+H 5

The channels (3) and (5) are also a result of an ISC mechanism, between the S1 and T128.

The molecular products CH4 and CO are obtained through channel (4) at wavelengths shorter than 248 nm, while the hydrogen atom elimination channels (3) and (5) emerge from the photodissociation of acetaldehyde at 205 nm 17. Channel (3) dominates over channel (5) due to the different transition energy values: 24.4 and 42.5 kcal-mol. Other reported channel 29, which leads to hydrogen neutral molecule elimination, is:

CH3CHOCH2CO+H2 6

All of the above processes, channels (2) to (6) can take place in one photon absorption regimes and the resulting neutral products have been reported. For instance the absorption of one-photon of 266 nm allows the neutral molecule reach the S1 state. If there is not an excess of energy transferred during the excitation, the S1 state can decay to S0 by an internal conversion processes IC, and form other products 29,30, including CH4 and CO.

In the present work, the photodissociation and photoionization of acetaldehyde in the multiple photon absorption regimes were investigated at wavelengths of 266 and 355 nm and intensities of radiation in the range 108 to 1011 W⋅cm-2. Radiation interacts with a cooled molecular beam of acetaldehyde produced by the adiabatic expansion of vapors into a high vacuum chamber at 10-8 torr. The resulting ions were analyzed using a home-assembled Jordan R-TOF mass analyzer.

On the basis of the detected ions at 266 and 355 nm, the processes were identified as D-I and I-D, respectively. The number of photons required to form a particular ion was calculated accordingly with the Keldysh approximation 31 and compared with the reported electronic energy levels 32,8. Along with those, based on the detected ions, the different dissociation pathways were proposed.

This work gives new insights on the molecular physical processes present when acetaldehyde molecules interact with photons and the products that can emerge as result of photoionization and photodissociation.

2. Experimental

The photoionization and photodissociation of acetaldehyde was analyzed using the experimental setup previously described 33. Briefly, a sample of acetaldehyde ((99%), purchased from Sigma Aldrich Chemical Corp) was used as received. The sample was connected to a pulsed injection valve, IOTA-ONE. A cooled molecular beam of acetaldehyde, with helium as a carrier gas was produced by adiabatic expansion in a high-vacuum chamber at 2 x 10-8torr. The pulsed valve was synchronously coupled with the laser pulses with an opening time of 250 μs to achieve an operating pressure of 2 x 10-6 torr. The 355 nm laser radiation was produced from the third harmonic of a Nd:YAG laser, operating at 30 Hz repetition rate (Spectra Physics). The laser pulse width is 5.5 ns and the energies per pulse from 1 to 30 mJ. When it was required, 266 nm photons were produced from the fourth harmonic by pumping a second crystal, frequency doubler, with 532 nm pulses of radiation. Thus, 266 nm photons with pulse widths of 3.5 nm and energies per pulse from 0.1 to 10 mJ were used. The laser radiation (with a Gaussian profile and linearly polarized) was focused into the interaction region using a 15 cm focal length lens. The diameter at the focal point was 80.0 μm. Under these experimental conditions, radiation intensities between 109 and