How should I play the half bass notes with the slashed stem?
See please the attached image:
It's called measured tremolo, and is a shorthand for writing a series of eighth notes. It means that you are to play repeated eighth notes that fill the time of the half note. So, for example, in the first measure, you'd play four eighth notes of A, followed by four eighth notes of G♯.
Just in case a total music nerd comes across this: a note stem with slash is the modern transcription of a "plica" in Modal (meaning: 12th/13th century School of Notre Dame) notation.
A plica is a single note with stems on both left and right side, both up or both down. The left stem has no meaning, but the right one indicates that the note is "plicated". This means that the note length (typically a longa, rendered as a quarter note) is split in two notes (therefore: eights), with the second a diatonic step up or down depending on the stem direction. In modern transcriptions, the stem of the second note receives a slash, to indicate that it was a plica in the original.
Example: https://imslp.org/wiki/Magnus_Liber_Organi_(Various) part 2, opening page, 3rd staff, 2nd note.