We present echelle long-slit optical spectra of a sample of objects evolving off the Asymptotic Giant Branch ( AGB ) , most of them in the pre-planetary nebula ( pPN ) phase , obtained with the ESI and MIKE spectrographs at the 10 m Keck II and 6.5 m Magellan-I telescopes , respectively .
The total wavelength range covered with ESI ( MIKE ) is \sim 3900 to 10900Ã Â ( \sim 3600 to 7200Ã ) .
In this paper , we focus our analysis mainly on the H \alpha  profiles .
Prominent H \alpha  emission is detected in half of the objects , most of which show broad H \alpha wings ( with total widths of up to \sim 4000 km s ^ { -1 } ) .
In the majority of the H \alpha -emission sources , fast , post-AGB winds are revealed by P-Cygni profiles .
In \sim 37 % of the objects H \alpha  is observed in absorption .
In almost all cases , the absorption profile is partially filled with emission , leading to complex , structured profiles that are interpreted as an indication of incipient post-AGB mass-loss .
The rest of the objects ( \sim 13 % ) are H \alpha  non-detections .
We investigate correlations between the H \alpha  profile and different stellar and envelope parameters .
All sources in which H \alpha  is seen mainly in absorption have F-G type central stars , whereas sources with intense H \alpha  emission span a larger range of spectral types from O to G , with a relative maximum around B , and also including very late C types .
Shocks may be an important excitation/ionization agent of the close stellar surroundings for objects with late type central stars .
Sources with pure emission or P Cygni H \alpha  profiles have larger J - K color excess than objects with H \alpha  mainly in absorption , which suggests the presence of warm dust near the star in the former .
The two classes of profile sources also segregate in the IRAS color-color diagram in a way that intense H \alpha -emitters have dust grains with a larger range of temperatures .
Spectral classification of the central stars in our sample is presented .
For a subsample ( 13 objects ) , the stellar luminosity has been derived from the analysis of the O i 7771-5Ã Â infrared triplet .
The location in the HR diagram of most of these targets , which represent \sim 30 % of the whole sample , is consistent with relatively high final ( and , presumably , initial ) masses in the range M _ { f } \sim 0.6-0.9 M _ { \odot } Â ( M _ { i } \sim 3-8 M _ { \odot } ) .