col
1   1
2   2
3   3

i   j   val
1   1    1
1   2    2
1   3    3
2   1    2
2   2    4
2   3    6
3   1    3
3   2    6
3   3    9

df2 = pd.DataFrame(index=pd.MultiIndex.from_product([df.index, df.col])).reset_index()
df2.columns = ['i', 'j']
df2['val'] = df2.i * df2.j

df2
Out[45]:
   i  j  val
0  1  1    1
1  1  2    2
2  1  3    3
3  2  1    2
4  2  2    4
5  2  3    6
6  3  1    3
7  3  2    6
8  3  3    9