Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Intrinsic pH of water/vapor interface revealed by ion-induced water
  alignment   Protons at the water/vapor interface are relevant for atmospheric and
environmental processes, yet to characterize their surface affinity on the
quantitative level is still challenging. Here we utilize phase-sensitive
sum-frequency vibrational spectroscopy to quantify the surface density of
protons (or their hydronium form) at the intrinsic water/vapor interface,
through inspecting the surface-field-induced alignment of water molecules in
the electrical double layer of ions. With hydrogen halides in water, the
surface adsorption of protons is found to be independent of specific
proton-halide anion interactions and to follow a constant adsorption free
energy, G about -3.74 (+/-0.56) kJ/mol, corresponding to a reduction of the
surface pH with respect to the bulk value by 0.66 (+/-0.10), for bulk ion
concentrations up to 0.3 M. Our spectroscopic study is not only of importance
in atmospheric chemistry, but also offers a microscopic-level basis to develop
advanced quantum-mechanical models for molecular simulations.

29
The categories of this paper are: funct-an
28
The categories of this paper are: alg-geom
8
The categories of this paper are: hep-ex
1
The categories of this paper are: cs
23
The categories of this paper are: solv-int
31
The categories of this paper are: ao-sci
16
The categories of this paper are: stat
36
The categories of this paper are: cmp-lg
12
The categories of this paper are: nucl-th
14
The categories of this paper are: math-ph
22
The categories of this paper are: chao-dyn
15
The categories of this paper are: nucl-ex
34
The categories of this paper are: chem-ph
19
The categories of this paper are: eess
33
The categories of this paper are: plasm-ph
24
The categories of this paper are: dg-ga