Scholars' Mine Scholars' Mine Femtosecond Laser Mass Spectrometry and High Harmonic Femtosecond Laser Mass Spectrometry and High Harmonic Spectroscopy of Xylene Isomers Spectroscopy of Xylene Isomers

, Structural isomers, molecules having the same chemical formula but with atoms bonded in different order, are hard to identify using conventional spectroscopy and mass spectrometry. They exhibit virtually indistinguishable mass spectra when ionized by electrons. Laser mass spectrometry based on photoionization of the isomers has emerged as a promising alternative but requires shaped ultrafast laser pulses. Here we use transform limited femtosecond pulses to distinguish the isomers using two methods. First, we probe doubly charged parent ions with circularly polarized light. We show that the yield of doubly charged ortho-xylene decreases while para-xylene increases over a range of laser intensities when the laser polarization is changed from linear to circular. Second, we probe high harmonic generation from randomly oriented isomer molecules subjected to an intense laser field. We show that the yield of high-order harmonics varies with the positioning of the methyl group in xylene isomers (ortho-, para- and meta-) and is due to differences in the strength of tunnel ionization and the overlap between the angular peaks of ionization and photo-recombination. A key task in analytical chemistry is to identify isomers – molecules that have the same chemical formula but have different chemical properties and reactivities. This is in part due to their critical role in pharmacology and medic-inal chemistry 1–3 . Isomeric identification techniques are either spectroscopic or spectrometric. Spectroscopy exploits specific transitions induced in isomers by electromagnetic radiation. UV-visible spectroscopy relies on electronic transitions to The harmonics were in a continuous flow jet formed from a nozzle of diameter 0.2 mm that was backed by ≈ 0.3 bar xylene vapour. Experiments were conducted by focusing mid-infrared pulses from the idler of an OPA (TOPAS HE, Lightconversion) centred at 1800 nm with a 50 cm CaF 2 lens into the 0.5 mm diameter gas jet. The OPA was pumped by 8 mJ, 800 nm, 25 fs pulses from a Ti:Sapphire laser operating (Red Dragon, KM Labs) at 1 kHz. The OPA beam was passed through an evacuated 400 μ m diameter hollow capillary acting as a spatial filter that ensured 0.6 mJ pulses of 50 fs dura-tion (confirmed by FROG measurement) in a HE 11 mode were delivered to the experiment. The focal spot was measured to have e − 2 radius 100 m, so the Rayleigh range was 17 mm and the generation occurred in the loose focusing limit. The HHG detection system was composed of a flat-field concave grating and a microchannel plate detector coupled to a phosphor screen and charge-coupled device (CCD) camera. The harmonics were produced in a non-saturated regime, confirmed by checking linear scaling of HHG cut-off with intensity, and for these measurements an intensity of around 3 × 10 14 Wcm − 2 was used. The ellipticity dependence was obtained by measuring the harmonics while rotating a quarter wave plate to 26 angles over a 36° range in random order. Reference measurements at linear polarization were interlaced to correct for laser drift. Error bars were obtained by bootstrapping with multiple angle scans (typically 3).