Do this:
In [43]: temp2.str[-1]
Out[43]:
0 p500
1 p600
2 p700
Name: ticker
So all together it would be:
>>> temp = pd.DataFrame({'ticker' : ['spx 5/25/2001 p500', 'spx 5/25/2001 p600', 'spx 5/25/2001 p700']})
>>> temp['ticker'].str.split(' ').str[-1]
0 p500
1 p600
2 p700
Name: ticker, dtype: object