1. Why is this paper interesting? -Relevant to the groups active research -Water is easily available in all phases -PACER measurements : D₂O/H₂O

2. Water HOMO - 2 HOMO - 1 HOMO 2.2eV 3.7eV Simple Molecule: HOMO, HOMO-1, HOMO-2 all identical p-type orbitals O₂ is not so simplistic.... HOMO separation; compare to CO₂..... 12.6eV

3. HHG in Molecules Electron wavepacket launched – orbital information Propagation in continuum – phase accumulation Dipole amplitudes interfere – phase & amplitude modulation

4. Experiment To date: HHG in H₂O observed using: 1) 800nm light with microdroplets 2) 3.6μm light with liquid This paper: Results from HHG in gas-phase H₂O 800nm & 1300nm light Intensities ≥ Isat Harmonics: 39 th at 800nm 91 st at 1300nm MOLECULES ARE NOT ALIGNED FOR THIS PAPER

5. Experimental Set-Up HHG 800nm light - Produced: 50cm Semi-Infinite Gas Cell (SIGC) - SIGC: 150μm pinhole - Ti:Saph Regen Amp (~3.5mJ, 40fs, 100Hz) - Focussed: f = 75cm achromatic lens - 2mm uncoated fused SiO₂ window Flat field concave grating → MCP →(Phosphor Screen) →CCD

6. Results -Note Xe cut-off points -Previous H₂O HHG work: Highest photon energy: ~42eV -This experiment: ~60eV -SIGC HHG efficiency: ~ - Extension of HHG cut off..... ? (a) Xe: Cut off: 106.8eV ? (a)H 2 O: Cut off: 107.2eV ?

7. Results -Harmonic spatial profiles nearly Gaussian in shape -Not the case beyond H₂O saturation intensity -Distortion due to plasma formation? -Need good spatial mode: extract harmonic phase and amplitude Higher photon energies & better phase matching – magnify suppression → extension of cut off harmonics

8. Results Xe and H₂O at 1300nm, I = Cut off energies: ~87eV for H₂O ~60eV for Xe Extension allows study of electronic and geometric structure – multiorbital and multicentre interferences start to appear in spectra at higher energies

9. Conclusions HHG demonstrated in H₂O: 800nm: ~60eV 1300nm: ~87eV Suppression of ionization shown to be due to orbital symmetry at tunnelling stage – comparing Xe and H₂O harmonics → Easy to align: study different orbital contribution to HHG HOMO separation supports this Interesting prospects with H₂O within the QOLS group