The McNab Group

<body> <h1> <font color="#CC0000">THE </font><font color="#FF6600">Mc</font><font color="#FFCC00">Na</font><font color="#00CC00">b G</font><font color="#3366FF">RO</font><font color="#CC33CC">UP</font></h1> <h2> Current Research Group consists of:</h2> <h3> Ramadan Abusen   <a href="http://www.ncl.ac.uk/~nirm/ramadan.htm"> click here for picture</a></h3> <h3> Simon G Cox</h3> <h3> Andrew D J Critchley <a href="http://www.ncl.ac.uk/~nirm/andy.htm">click here for picture</a></h3> <h3> Alan N Hughes</h3> <h3> Christine A Leach (Honorary Research Fellow)</h3> <h3> <a href="MAILTO:iain.mcnab@ncl.ac.uk">Iain R McNab</a>  (Boss) <a href="http://www.ncl.ac.uk/~nirm/irmcar.htm">click here for picture</a></h3> <h3> Fiona E Smith</h3> <h3> John Walley (occasional employee!) <a href="http://www.walley.freeserve.co.uk/">click here for John's website</a></h3> <h2> Research Group Alumni:</h2> <h3> Dr Ralph C Shiell</h3> <hr> <h2> Postal address:</h2> Department of Physics, <br>University of Newcastle upon Tyne, <br>Newcastle upon Tyne NE1 7RU, <br>United Kingdom. <h2> Phone numbers:</h2> Iain McNab (Office)           (+44) 0191 222 7408 <br>Group and Laboratory        (+44) 0191 222 8881 <h3> <hr></h3> <h1> Index</h1> <ul> <li> <a href="#research">Research interests</a></li> <li> <a href="#overview">Overview of Research Program</a></li> <li> <a href="http://www.ncl.ac.uk/~nirm/pubs.htm">Publications</a></li> <li> <a href="http://www.ncl.ac.uk/~nirm/irmcv.htm">Curriculum vitae</a></li> <li> <a href="#fr">Summary of recent research</a>  <hr><a NAME="research"></a></li> <h2> Research Interests</h2> <ol> <li> Spectroscopy of Molecular Dications</li> <br>Our research uses a combination of infrared laser spectroscopy and mass spectrometry to obtain structural information about simple molecules with a double positive charge (molecular dications). We have obtained high resolution infrared measurements of a <a href="#dicat">molecular dication</a> (DCl2+). This work was described as a "world first" by the EPSRC . We are also interested in the <a href="#hyp">hyperfine structure</a> of molecules which arises due to interactions of various intrinsic angular momenta within the molecule. For DCl++ we have succeeded in obtaining hyperfine information. The first hyperfine information ever to be measured for a molecular dication.</ol> <ol> <li> Electric Field Dissociation of Molecular Ions</li> <br>Electric fields tear apart molecular ions. This effect is well known, but has been little studied. Our research uses a ZAB-HF double focusing mass spectrometer to investigate the applications of electric field dissociation to mass spectrometry.</ol> <hr> <h2> <a NAME="overview"></a>Overview of Research Program</h2> The determination of the structure of simple molecular ions yields data which provides a rigorous test of molecular quantum mechanics [1]. Currently <i>ab-initio</i> calculations of the properties of double-charged ions (dications) are difficult to perform, and there is little experimental data with which to compare them. We are interested in their structure both as a test of molecular quantum mechanics, and because dications embedded in a matrix of inert-gas form a possible route to high energy-density storage devices for use in satellites. <p>The properties of molecular dications are difficult to calculate. For light dications, the dissociation limit corresponds to two charged fragments repelling each other due to the coulomb energy. As the fragments are brought closer together the chemical bond lowers the energy and leads to a potential energy well which supports vibration-rotation levels. Such levels are all quasibound, because they lie above the dissociation limit. The lifetimes of the deepest levels may be days or hours, while the uppermost levels may fragment in picoseconds. <p>We obtain high resolution vibration-rotation spectra of dications by interacting a molecular ion beam formed in a modified mass spectrometer with the beam from an infrared laser. Despite being unstable, dications can be very long-lived (up to hours), and the decay rate is different in different energy levels. Transitions will be driven between metastable levels with different decay rates, and the absorption of radiation detected by the change in fragmentation rate upon population transfer. Fragments are formed at different kinetic energy releases, and by using kinetic energy analyzer before counting the number of fragments, we can select only those of interest. <p>This work is supported be the EPSRC. Our experiment has detected a vibration-rotation spectrum of the dication of HCl. This is only the third molecular dication for which such high resolution data are available and analysis of the spectrum will yield hitherto unavailable information on molecular dications and provide rigorous tests of molecular quantum mechanics. <ol> <li> <b>An infrared spectrum of the molecular dication (doubly positively charged molecule), D35Cl++</b> A communication <i>Journal of Chemical Physics</i>, volume 108, pages 1761-1764 (1998), R. Abusen, F.R. Bennett, I.R. McNab, D.N. Sharp, R.C. Shiell & C.A. Woodward</li> <br><b>The Theoretical Infrared Spectrum of HCl2+ and its isotopomers</b> In <i>Chemical physics letters</i>, 251,405-412 (1996), F.R. Bennett and I.R. McNab.</ol> <p><a NAME="fr"></a> <h1> Recent Research</h1> <h2> Contents</h2> <ol> <li> <a href="#intro">Introduction</a></li> <li> <a href="#achiev">Achievements of the research</a></li> <ul> <li> <a href="#outline">Outline of experiment</a></li> <li> <a href="#mass">The mass spectrometry of DCl2+</a></li> <li> <a href="#infrared">The infrared spectrum of DCl2+</a></li> <li> <a href="#assign">Assignment of the spectrum</a></li> </ul> <li> <a href="#conclude">Conclusion</a></li> </ol> <a NAME="intro"></a> <p>INTRODUCTION <p>Spectra of molecular dications (doubly positively charged molecules) with better than vibrational resolution have proved difficult to obtain. The first dication to be measured with rotational resolution was N22+ (1958), the second was NO2+ (1987) and our measurements of DCl2+ (1995) are the third. The timescales involved give a good idea of the experimental difficulties involved in determining highly resolved spectra of these species. <a NAME="dicat"></a> <p>Molecular dications are metastable and extremely chemically reactive and are the subject of growing interest. The molecular bonding causes unusual potential energy (hereafter pe) curves that have a minimum and a barrier to dissociation into two charged fragments;examples of such curves are shown in figure one. Vibrational levels, all of which are metastable, may be supported above the dissociation limit. The lower levels can be extremely long lived, whereas the upper levels may dissociate within picoseconds. If population is transferred to such levels the resulting "Coulomb explosion" releases considerable kinetic energy (several eV / dication) and dications have been proposed as a source of propulsion. <p>The shape of dication pe curves suggests that the barrier may result from an avoided crossing but sophisticated calculations show that it is better thought of as resulting from competition between Coulomb and valence forces. It was initially believed that dications would dissociate by tunneling through the barrier (which would cause J-dependent linewidths), but for N22+ measurements show that in the lower vibrational levels of the first excited electronic state the dissociation is caused by electronic predissociation (with no J-dependence of linewidths). Ab-initio calculations are not yet able to predict the observed structure and lifetimes of dications and it seems that the region close to the top of the barrier (which is that probed by fast ion beam spectroscopy) is the most demanding part of the pe curve to calculate accurately. We aim to probe the nature of dications to provide answers to general questions: What is the nature of the bonding in these molecules? What is their structure? Which molecules dissociate via electronic predissociation and which by tunneling; this is equivalent to asking what is the nature of the angular momentum specific half collision? <a NAME="achiev"></a> <p>ACHIEVEMENTS OF THE RESEARCH <p>Our most important objective, to obtain dication spectra, has been achieved; we have now observed many vbration rotation transitions of the doubly charged molecule DCl2+. As theoretical support we have calculated accurate ab initio pe functions for the lowest four electronic states of DCl2+ which will be adjusted until agreement with experimental data is achieved. <p><a NAME="outline"></a> <h2> Outline of Experiment</h2> In the new fast ion beam spectrometer ions are formed in an electron impact ion source and accelerated into a magnetic sector where the dication of interest is selected. The selected dications form a beam which is collinear with a laser beam from a line tuneable CO/CO2 laser (lines typically separated by 1-2 cm-1). For light dications almost complete frequency coverage can be obtained by Doppler shifting the ions using retarding or accelerating voltages applied to the "drift tube". At resonance, population is moved from one state to another with a different predissociation lifetime, consequently the number of fragment ions formed is increased. The fragment ions are selected with an electrostatic analyzer and detected with an electron multiplier. A plot of fragment ion intensity against Doppler shifted frequency yields the spectrum. <p>We decided that the ideal candidate dication for study would have one light and one heavy nucleus. A because the fragments which are monitored with the electrostatic analyzer are far removed from the parent peak. The consequence of the above equations is that a dication which fragments to one light and one heavy fragment will display three convenient properties. (i) Although the fragments have half the charge of the parent, the masses are not close to half the parent mass and hence will be transmitted at far removed electrostatic analyzer voltages; this avoids the problem of overlapping the parent beam. (ii) The resolution of the analyzer is fixed and given by `E/E, because the light fragment forward-backward scattering pattern appears at low energy (small E) it is well resolved and this aids kinetic energy analysis. (iii) The heavy fragment is not well resolved by the analyzer and so shows essentially no splitting; the resulting high collection efficiency presents an ideal way of monitoring increased fragmentation on resonance. <p>With these considerations in mind we decided to examine the species DCl2+, a light- heavy diatomic with many predicted transitions in our wavenumber range (HCl2+ only has one predicted transition). <h3> <a NAME="mass"></a>The mass spectrometry of DCl2+.</h3> The magnetic analyzer following the ion source transmits ions with a specific mass/charge ratio. We can be certain that we are examining DCl2+ because both the isotope D35Cl2+ and D37Cl2+ have odd masses and hence appear at half integer mass/charge (37/2 and 39/2). In addition to this, the expected pattern of intensity is 3:1 for the 35Cl and 37Cl isotopes respectively and this is clearly observed. <h3> <a NAME="infrared"></a>The infrared spectrum of DCl2+.</h3> Our recent calculations led us to believe that the infra-red spectrum of DCl2+ in its ground electronic state (triplet sigma) should be fairly extensive within the wavenumber and lifetime ranges available to us. These calculations agreed quite closely with previous (less sophisticated) ab-initio work, but appear to be at odds with the recent photoelectron spectra obtained by McConkey et al.. Their photoelectron spectrum of HCl2+ is vibrationally resolved and appears to show five vibrational levels in the ground electronic state. Our calculations predict only three vibrational levels. Normally one would assume that the calculations are in error, but the possibility that the photoelectron work is mast-assigned is strengthened by the fact that the photoelectron spectrum contains a further unassigned peak to high wavenumber of the assigned vibrational progression. DCl2+ is therefore a doubly- charged molecule about which there is currently some contention, a situation we hope to resolve. <p>We have now recorded hundreds of vibration-rotation transition in D35Cl2+ and D37Cl2+ and we are searching for more. The complete recording and analysis of the infrared spectrum of DCl2+ is in progress and will be submitted to Physical Review Letters in due course. A preliminary communication was published as a Journal of Chemical Physics Communication (108, 1761-1764 (1998). In order to be certain that the first transitions we found were genuine we confirmed that: <p>(a) that the transition disappears when the laser is off (it does), <p>(b) that the transitions disappears when adjacent laser lines are used (it does), <p>(c) that the transition is observable at different combinations of accelerating and <p>retarding voltages (it is), <p>(d) that the transition can be observed by monitoring both D+ and Cl+ fragments (it can). <p>(e) that the "on-resonance" kinetic energy release spectrum shows a centre of mass kinetic energy release appropriate for the ground state (it does). <h3> <a NAME="assign"></a>Assignment of the spectrum</h3> The analysis of the infrared spectrum of DCl2+ should be straightforward as we have four "tools" with which to aid our spectroscopic assignment: linewidths (tunneling lifetimes), rotational progressions, vibration-rotation isotope effects, and measurements of the kinetic energy of fragmentation. <p>Theory shows that the tunneling lifetimes of dications are less sensitive to rotational level than those of normal molecules. For DCl2+ the lifetime changes from 7x10-8s at J=10 to 2x10-10s at J=20. The insensitivity to angular momentum arises because the potential barrier is mostly due to Coulomb repulsion between the two charge centers. The practical result of this is that we expect to be able to observe rotational progressions of lines (one is not usually so fortunate in fast ion beam work) which will be amenable to traditional methods of analysis. <p>The tunneling lifetimes of the states involved in these spectroscopic transitions determine the measured linewidths. Calculations show that a small change in the vibration- rotation energy of the upper state results in extremely small changes in the calculated lifetimes. Consequently, we expect to use the measured linewidths as an aid in our initial assignment; the widths should be a good guide to the barrier width (and hence energy beneath the top of the potential barrier) confining the upper level of the transition. <p>Each transition that is measured releases fragments with considerable kinetic energy. The geometry of the instrument is such that only fragments scattered within 3 milli-steradians about the beam axis are detected. The resultant forward-backward scattering pattern for D+ fragments from DCl2+ is shown in figure four. By narrowing the acceptance angle of the ESA the resolution can be increased sufficiently to enable the energy release to be compared with calculations of absolute vibration-rotation energy levels as an aid to assignment. <p>Isotopic substitution shifts vibration-rotation transition wavenumbers in a well understood manner. DCl2+ is available in two isotopes (the natural abundance ratios for Chlorine result in an observed 3:1 mix of D35Cl2+: D37Cl2+ in the ion beam) and we have recorded spectra of both. As the difference between the reduced masses is small, we observe pairs of lines (one for each isotope). The measured shift between isotopes is direct evidence of the vibration-rotation band involved. <p>We do not yet have sufficient data to assign the rotational structure unambiguously, but we are certain that we are measuring transitions in the 2-1 vibrational band. The transitions are very intense, and vary in intensity by over four orders of magnitude. We can easily see the strongest transitions using laser powers of a few milli Watts, and arrangements are in progress to attempt to record the spectrum with diode lasers. <p>Many of the lines that we see in the spectrum are in a characteristic grouping of four closely spaced transitions. This is due to <a NAME="hyp"></a>hyperfine structure. Such structure arises because of the interactions between electron spin (S), the rotation of the nuclear framework (R), the rotation of the electrons (L) and the nuclear spins (ID=1, ICl=3/2); the largest hyperfine interaction is probably the fermi contact-interaction, which we estimate to be 200 MHz. We have programmed a perturbation Hamiltonian with which to calculate this hyperfine structure based on the triplet sigma hamiltonian of Carrington et al.. Because it is possible to resolve the hyperfine structure in some lines of the spectrum we expect to learn more about the molecular physics of the bond in DCl2+ than has hitherto been possible for any molecular dication. <p><a NAME="conclude"></a>CONCLUSION <p>We have located the first infrared spectrum of a molecular dication. We are confident that we can now greatly extend high resolution experimental data on molecular dications and use the data to improve current levels of understanding.</ul> </body>